DateTime Calculations - Java Programming

When calculating ages, finding days until deadlines, or scheduling future events, you need date arithmetic. Java provides Period for date-based amounts and Duration for time-based amounts, making these calculations precise and readable.

DateTime calculations perform arithmetic with dates and times. Java uses Period for date-based amounts and Duration for time-based amounts.

Date Arithmetic

Arithmetic.java

// Add and subtract dates

import java.time.LocalDate;
import java.time.LocalDateTime;
import java.time.Period;
import java.time.temporal.ChronoUnit;

public class Arithmetic {

    public static void main(String[] args) {
        LocalDate today = ;
        System.out.println("Today: " + today);
        System.out.println();

        // Add days
        System.out.println("Add days:");
        System.out.println("Tomorrow: " + today.plusDays(1));
        System.out.println("+7 days: " + today.plusDays(7));
        System.out.println("+30 days: " + today.plusDays(30));

        // Add weeks
        System.out.println("\nAdd weeks:");
        System.out.println("+1 week: " + today.plusWeeks(1));
        System.out.println("+4 weeks: " + today.plusWeeks(4));

        // Add months
        System.out.println("\nAdd months:");
        System.out.println("+1 month: " + today.plusMonths(1));
        System.out.println("+3 months: " + today.plusMonths(3));
        System.out.println("+12 months: " + today.plusMonths(12));

        // Add years
        System.out.println("\nAdd years:");
        System.out.println("+1 year: " + today.plusYears(1));
        System.out.println("+5 years: " + today.plusYears(5));

        // Subtract
        System.out.println("\nSubtract:");
        System.out.println("Yesterday: " + today.minusDays(1));
        System.out.println("-1 week: " + today.minusWeeks(1));
        System.out.println("-1 month: " + today.minusMonths(1));
        System.out.println("-1 year: " + today.minusYears(1));

        // Chaining
        System.out.println("\nChaining:");
        LocalDate future = today.plusMonths(3).plusDays(15);
        System.out.println("+3 months +15 days: " + future);

        // Period
        System.out.println("\nUsing Period:");
        Period p1 = Period.ofDays(7);
        System.out.println("Period of 7 days: " + today.plus(p1));
        
        Period p2 = Period.ofMonths(2);
        System.out.println("Period of 2 months: " + today.plus(p2));
        
        Period p3 = Period.of(1, 3, 15);  // 1 year, 3 months, 15 days
        System.out.println("Period 1y 3m 15d: " + today.plus(p3));

        // DateTime arithmetic
        System.out.println("\nDateTime arithmetic:");
        LocalDateTime now = LocalDateTime.of(2025, 1, 29, 14, 30, 0);
        System.out.println("Now: " + now);
        System.out.println("+2 hours: " + now.plusHours(2));
        System.out.println("+30 minutes: " + now.plusMinutes(30));
        System.out.println("+1 day 2 hours: " + now.plusDays(1).plusHours(2));

        // End of periods
        System.out.println("\nEnd of periods:");
        System.out.println("End of week: " + today.plusWeeks(1));
        System.out.println("End of month: " + today.plusMonths(1));
        System.out.println("End of quarter: " + today.plusMonths(3));
        System.out.println("End of year: " + today.plusYears(1));

        // Practical examples
        System.out.println("\nPractical examples:");
        System.out.println("30-day trial ends: " + today.plusDays(30));
        System.out.println("Quarterly review: " + today.plusMonths(3));
        System.out.println("Annual renewal: " + today.plusYears(1));
        System.out.println("Two weeks notice: " + today.plusWeeks(2));
    }

}

// Add and subtract dates

import java.time.LocalDate;
import java.time.LocalDateTime;
import java.time.Period;
import java.time.temporal.ChronoUnit;

public class Arithmetic {

    public static void main(String[] args) {
        LocalDate today = ;
        System.out.println("Today: " + today);
        System.out.println();

        // Add days
        System.out.println("Add days:");
        System.out.println("Tomorrow: " + today.plusDays(1));
        System.out.println("+7 days: " + today.plusDays(7));
        System.out.println("+30 days: " + today.plusDays(30));

        // Add weeks
        System.out.println("\nAdd weeks:");
        System.out.println("+1 week: " + today.plusWeeks(1));
        System.out.println("+4 weeks: " + today.plusWeeks(4));

        // Add months
        System.out.println("\nAdd months:");
        System.out.println("+1 month: " + today.plusMonths(1));
        System.out.println("+3 months: " + today.plusMonths(3));
        System.out.println("+12 months: " + today.plusMonths(12));

        // Add years
        System.out.println("\nAdd years:");
        System.out.println("+1 year: " + today.plusYears(1));
        System.out.println("+5 years: " + today.plusYears(5));

        // Subtract
        System.out.println("\nSubtract:");
        System.out.println("Yesterday: " + today.minusDays(1));
        System.out.println("-1 week: " + today.minusWeeks(1));
        System.out.println("-1 month: " + today.minusMonths(1));
        System.out.println("-1 year: " + today.minusYears(1));

        // Chaining
        System.out.println("\nChaining:");
        LocalDate future = today.plusMonths(3).plusDays(15);
        System.out.println("+3 months +15 days: " + future);

        // Period
        System.out.println("\nUsing Period:");
        Period p1 = Period.ofDays(7);
        System.out.println("Period of 7 days: " + today.plus(p1));
        
        Period p2 = Period.ofMonths(2);
        System.out.println("Period of 2 months: " + today.plus(p2));
        
        Period p3 = Period.of(1, 3, 15);  // 1 year, 3 months, 15 days
        System.out.println("Period 1y 3m 15d: " + today.plus(p3));

        // DateTime arithmetic
        System.out.println("\nDateTime arithmetic:");
        LocalDateTime now = LocalDateTime.of(2025, 1, 29, 14, 30, 0);
        System.out.println("Now: " + now);
        System.out.println("+2 hours: " + now.plusHours(2));
        System.out.println("+30 minutes: " + now.plusMinutes(30));
        System.out.println("+1 day 2 hours: " + now.plusDays(1).plusHours(2));

        // End of periods
        System.out.println("\nEnd of periods:");
        System.out.println("End of week: " + today.plusWeeks(1));
        System.out.println("End of month: " + today.plusMonths(1));
        System.out.println("End of quarter: " + today.plusMonths(3));
        System.out.println("End of year: " + today.plusYears(1));

        // Practical examples
        System.out.println("\nPractical examples:");
        System.out.println("30-day trial ends: " + today.plusDays(30));
        System.out.println("Quarterly review: " + today.plusMonths(3));
        System.out.println("Annual renewal: " + today.plusYears(1));
        System.out.println("Two weeks notice: " + today.plusWeeks(2));
    }

}

Period Represents a date-based amount of time in years, months, and days. Used for calendar calculations.

Calculating Differences

Difference.java

// Calculate differences

import java.time.LocalDate;
import java.time.LocalDateTime;
import java.time.Period;
import java.time.Duration;
import java.time.temporal.ChronoUnit;

public class Difference {

    public static void main(String[] args) {
        LocalDate start = LocalDate.of(2025, 1, 1);
        LocalDate end = ;
        
        System.out.println("Start: " + start);
        System.out.println("End: " + end);
        System.out.println();

        // Period between
        Period period = Period.between(start, end);
        System.out.println("Period between:");
        System.out.println("Years: " + period.getYears());
        System.out.println("Months: " + period.getMonths());
        System.out.println("Days: " + period.getDays());
        System.out.println("Full period: " + period);

        // Total days
        long days = ChronoUnit.DAYS.between(start, end);
        System.out.println("\nTotal days: " + days);

        // Other units
        System.out.println("\nOther units:");
        System.out.println("Weeks: " + ChronoUnit.WEEKS.between(start, end));
        System.out.println("Months: " + ChronoUnit.MONTHS.between(start, end));
        System.out.println("Years: " + ChronoUnit.YEARS.between(start, end));

        // Different date pairs
        System.out.println("\nDifferent pairs:");
        LocalDate d1 = LocalDate.of(2020, 1, 15);
        LocalDate d2 = LocalDate.of(2025, 3, 20);
        
        Period p = Period.between(d1, d2);
        System.out.println(d1 + " to " + d2);
        System.out.println("Period: " + p.getYears() + "y " + p.getMonths() + "m " + p.getDays() + "d");
        System.out.println("Total days: " + ChronoUnit.DAYS.between(d1, d2));

        // DateTime differences
        System.out.println("\nDateTime differences:");
        LocalDateTime dt1 = LocalDateTime.of(2025, 1, 29, 10, 0);
        LocalDateTime dt2 = LocalDateTime.of(2025, 1, 29, 15, 30);
        
        Duration duration = Duration.between(dt1, dt2);
        System.out.println(dt1 + " to " + dt2);
        System.out.println("Duration: " + duration);
        System.out.println("Hours: " + duration.toHours());
        System.out.println("Minutes: " + duration.toMinutes());
        System.out.println("Seconds: " + duration.toSeconds());

        // Time units
        System.out.println("\nTime units between:");
        System.out.println("Hours: " + ChronoUnit.HOURS.between(dt1, dt2));
        System.out.println("Minutes: " + ChronoUnit.MINUTES.between(dt1, dt2));
        System.out.println("Seconds: " + ChronoUnit.SECONDS.between(dt1, dt2));

        // Absolute difference
        System.out.println("\nAbsolute difference:");
        LocalDate past = LocalDate.of(2020, 1, 1);
        LocalDate future = LocalDate.of(2030, 1, 1);
        
        System.out.println("Past to future: " + ChronoUnit.DAYS.between(past, future));
        System.out.println("Future to past: " + ChronoUnit.DAYS.between(future, past));
        System.out.println("Absolute: " + Math.abs(ChronoUnit.DAYS.between(future, past)));

        // Practical examples
        System.out.println("\nPractical examples:");
        
        LocalDate birthday = LocalDate.of(1990, 3, 15);
        LocalDate today = LocalDate.of(2025, 1, 29);
        long age = ChronoUnit.YEARS.between(birthday, today);
        System.out.println("Age: " + age + " years");
        
        LocalDate projectStart = LocalDate.of(2025, 1, 1);
        LocalDate projectEnd = LocalDate.of(2025, 3, 31);
        long projectDays = ChronoUnit.DAYS.between(projectStart, projectEnd);
        System.out.println("Project duration: " + projectDays + " days");
        
        LocalDate subscriptionStart = LocalDate.of(2024, 1, 1);
        LocalDate subscriptionEnd = LocalDate.of(2025, 1, 1);
        long subscriptionMonths = ChronoUnit.MONTHS.between(subscriptionStart, subscriptionEnd);
        System.out.println("Subscription: " + subscriptionMonths + " months");
    }

}

// Calculate differences

import java.time.LocalDate;
import java.time.LocalDateTime;
import java.time.Period;
import java.time.Duration;
import java.time.temporal.ChronoUnit;

public class Difference {

    public static void main(String[] args) {
        LocalDate start = LocalDate.of(2025, 1, 1);
        LocalDate end = ;
        
        System.out.println("Start: " + start);
        System.out.println("End: " + end);
        System.out.println();

        // Period between
        Period period = Period.between(start, end);
        System.out.println("Period between:");
        System.out.println("Years: " + period.getYears());
        System.out.println("Months: " + period.getMonths());
        System.out.println("Days: " + period.getDays());
        System.out.println("Full period: " + period);

        // Total days
        long days = ChronoUnit.DAYS.between(start, end);
        System.out.println("\nTotal days: " + days);

        // Other units
        System.out.println("\nOther units:");
        System.out.println("Weeks: " + ChronoUnit.WEEKS.between(start, end));
        System.out.println("Months: " + ChronoUnit.MONTHS.between(start, end));
        System.out.println("Years: " + ChronoUnit.YEARS.between(start, end));

        // Different date pairs
        System.out.println("\nDifferent pairs:");
        LocalDate d1 = LocalDate.of(2020, 1, 15);
        LocalDate d2 = LocalDate.of(2025, 3, 20);
        
        Period p = Period.between(d1, d2);
        System.out.println(d1 + " to " + d2);
        System.out.println("Period: " + p.getYears() + "y " + p.getMonths() + "m " + p.getDays() + "d");
        System.out.println("Total days: " + ChronoUnit.DAYS.between(d1, d2));

        // DateTime differences
        System.out.println("\nDateTime differences:");
        LocalDateTime dt1 = LocalDateTime.of(2025, 1, 29, 10, 0);
        LocalDateTime dt2 = LocalDateTime.of(2025, 1, 29, 15, 30);
        
        Duration duration = Duration.between(dt1, dt2);
        System.out.println(dt1 + " to " + dt2);
        System.out.println("Duration: " + duration);
        System.out.println("Hours: " + duration.toHours());
        System.out.println("Minutes: " + duration.toMinutes());
        System.out.println("Seconds: " + duration.toSeconds());

        // Time units
        System.out.println("\nTime units between:");
        System.out.println("Hours: " + ChronoUnit.HOURS.between(dt1, dt2));
        System.out.println("Minutes: " + ChronoUnit.MINUTES.between(dt1, dt2));
        System.out.println("Seconds: " + ChronoUnit.SECONDS.between(dt1, dt2));

        // Absolute difference
        System.out.println("\nAbsolute difference:");
        LocalDate past = LocalDate.of(2020, 1, 1);
        LocalDate future = LocalDate.of(2030, 1, 1);
        
        System.out.println("Past to future: " + ChronoUnit.DAYS.between(past, future));
        System.out.println("Future to past: " + ChronoUnit.DAYS.between(future, past));
        System.out.println("Absolute: " + Math.abs(ChronoUnit.DAYS.between(future, past)));

        // Practical examples
        System.out.println("\nPractical examples:");
        
        LocalDate birthday = LocalDate.of(1990, 3, 15);
        LocalDate today = LocalDate.of(2025, 1, 29);
        long age = ChronoUnit.YEARS.between(birthday, today);
        System.out.println("Age: " + age + " years");
        
        LocalDate projectStart = LocalDate.of(2025, 1, 1);
        LocalDate projectEnd = LocalDate.of(2025, 3, 31);
        long projectDays = ChronoUnit.DAYS.between(projectStart, projectEnd);
        System.out.println("Project duration: " + projectDays + " days");
        
        LocalDate subscriptionStart = LocalDate.of(2024, 1, 1);
        LocalDate subscriptionEnd = LocalDate.of(2025, 1, 1);
        long subscriptionMonths = ChronoUnit.MONTHS.between(subscriptionStart, subscriptionEnd);
        System.out.println("Subscription: " + subscriptionMonths + " months");
    }

}

Duration Represents a time-based amount in seconds and nanoseconds. Used for precise time measurements.

Age Calculation

Age.java

// Age calculation

import java.time.LocalDate;
import java.time.Period;
import java.time.temporal.ChronoUnit;

public class Age {

    public static void main(String[] args) {
        LocalDate birthdate = ;
        LocalDate today = LocalDate.of(2025, 1, 29);
        
        System.out.println("Birthdate: " + birthdate);
        System.out.println("Today: " + today);
        System.out.println();

        // Age in years
        long ageYears = ChronoUnit.YEARS.between(birthdate, today);
        System.out.println("Age (years): " + ageYears);

        // Age as period
        Period age = Period.between(birthdate, today);
        System.out.println("Age (period): " + age.getYears() + " years, " + 
                          age.getMonths() + " months, " + age.getDays() + " days");

        // Age in different units
        System.out.println("\nAge in different units:");
        System.out.println("Years: " + ChronoUnit.YEARS.between(birthdate, today));
        System.out.println("Months: " + ChronoUnit.MONTHS.between(birthdate, today));
        System.out.println("Days: " + ChronoUnit.DAYS.between(birthdate, today));

        // Next birthday
        LocalDate nextBirthday = birthdate.withYear(today.getYear());
        if (nextBirthday.isBefore(today) || nextBirthday.isEqual(today)) {
            nextBirthday = nextBirthday.plusYears(1);
        }
        System.out.println("\nNext birthday: " + nextBirthday);
        System.out.println("Days until birthday: " + ChronoUnit.DAYS.between(today, nextBirthday));

        // Age at specific date
        System.out.println("\nAge at specific dates:");
        LocalDate[] checkDates = {
            LocalDate.of(2020, 1, 1),
            LocalDate.of(2025, 1, 1),
            LocalDate.of(2030, 12, 31)
        };
        
        for (LocalDate checkDate : checkDates) {
            long ageAt = ChronoUnit.YEARS.between(birthdate, checkDate);
            System.out.println("Age on " + checkDate + ": " + ageAt);
        }

        // Multiple people
        System.out.println("\nMultiple people:");
        LocalDate[][] people = {
            {LocalDate.of(1990, 3, 15), today},
            {LocalDate.of(1985, 7, 22), today},
            {LocalDate.of(2000, 11, 30), today}
        };
        
        for (int i = 0; i < people.length; i++) {
            LocalDate birth = people[i][0];
            LocalDate current = people[i][1];
            Period p = Period.between(birth, current);
            System.out.println("Person " + (i + 1) + " (born " + birth + "): " + 
                             p.getYears() + "y " + p.getMonths() + "m " + p.getDays() + "d");
        }

        // Age verification
        System.out.println("\nAge verification:");
        System.out.println("Is 18 or older? " + isAdult(birthdate, today));
        System.out.println("Is 21 or older? " + isMinimumAge(birthdate, today, 21));
        System.out.println("Is senior (65+)? " + isMinimumAge(birthdate, today, 65));

        // Age groups
        System.out.println("\nAge group:");
        System.out.println(getAgeGroup(birthdate, today));

        // Time until milestone
        System.out.println("\nMilestone birthdays:");
        int[] milestones = {18, 21, 30, 40, 50};
        int currentAge = (int) ChronoUnit.YEARS.between(birthdate, today);
        
        for (int milestone : milestones) {
            if (milestone > currentAge) {
                LocalDate milestoneDate = birthdate.plusYears(milestone);
                long daysUntil = ChronoUnit.DAYS.between(today, milestoneDate);
                System.out.println(milestone + " years: " + milestoneDate + 
                                 " (in " + daysUntil + " days)");
            }
        }

        // Retirement calculation
        System.out.println("\nRetirement:");
        int retirementAge = 65;
        LocalDate retirementDate = birthdate.plusYears(retirementAge);
        long yearsToRetirement = ChronoUnit.YEARS.between(today, retirementDate);
        long daysToRetirement = ChronoUnit.DAYS.between(today, retirementDate);
        System.out.println("Retirement date: " + retirementDate);
        System.out.println("Years to retirement: " + yearsToRetirement);
        System.out.println("Days to retirement: " + daysToRetirement);
    }

    public static boolean isAdult(LocalDate birthdate, LocalDate currentDate) {
        return ChronoUnit.YEARS.between(birthdate, currentDate) >= 18;
    }

    public static boolean isMinimumAge(LocalDate birthdate, LocalDate currentDate, int minAge) {
        return ChronoUnit.YEARS.between(birthdate, currentDate) >= minAge;
    }

    public static String getAgeGroup(LocalDate birthdate, LocalDate currentDate) {
        long age = ChronoUnit.YEARS.between(birthdate, currentDate);
        if (age < 13) return "Child";
        if (age < 18) return "Teenager";
        if (age < 65) return "Adult";
        return "Senior";
    }

}

// Age calculation

import java.time.LocalDate;
import java.time.Period;
import java.time.temporal.ChronoUnit;

public class Age {

    public static void main(String[] args) {
        LocalDate birthdate = ;
        LocalDate today = LocalDate.of(2025, 1, 29);
        
        System.out.println("Birthdate: " + birthdate);
        System.out.println("Today: " + today);
        System.out.println();

        // Age in years
        long ageYears = ChronoUnit.YEARS.between(birthdate, today);
        System.out.println("Age (years): " + ageYears);

        // Age as period
        Period age = Period.between(birthdate, today);
        System.out.println("Age (period): " + age.getYears() + " years, " + 
                          age.getMonths() + " months, " + age.getDays() + " days");

        // Age in different units
        System.out.println("\nAge in different units:");
        System.out.println("Years: " + ChronoUnit.YEARS.between(birthdate, today));
        System.out.println("Months: " + ChronoUnit.MONTHS.between(birthdate, today));
        System.out.println("Days: " + ChronoUnit.DAYS.between(birthdate, today));

        // Next birthday
        LocalDate nextBirthday = birthdate.withYear(today.getYear());
        if (nextBirthday.isBefore(today) || nextBirthday.isEqual(today)) {
            nextBirthday = nextBirthday.plusYears(1);
        }
        System.out.println("\nNext birthday: " + nextBirthday);
        System.out.println("Days until birthday: " + ChronoUnit.DAYS.between(today, nextBirthday));

        // Age at specific date
        System.out.println("\nAge at specific dates:");
        LocalDate[] checkDates = {
            LocalDate.of(2020, 1, 1),
            LocalDate.of(2025, 1, 1),
            LocalDate.of(2030, 12, 31)
        };
        
        for (LocalDate checkDate : checkDates) {
            long ageAt = ChronoUnit.YEARS.between(birthdate, checkDate);
            System.out.println("Age on " + checkDate + ": " + ageAt);
        }

        // Multiple people
        System.out.println("\nMultiple people:");
        LocalDate[][] people = {
            {LocalDate.of(1990, 3, 15), today},
            {LocalDate.of(1985, 7, 22), today},
            {LocalDate.of(2000, 11, 30), today}
        };
        
        for (int i = 0; i < people.length; i++) {
            LocalDate birth = people[i][0];
            LocalDate current = people[i][1];
            Period p = Period.between(birth, current);
            System.out.println("Person " + (i + 1) + " (born " + birth + "): " + 
                             p.getYears() + "y " + p.getMonths() + "m " + p.getDays() + "d");
        }

        // Age verification
        System.out.println("\nAge verification:");
        System.out.println("Is 18 or older? " + isAdult(birthdate, today));
        System.out.println("Is 21 or older? " + isMinimumAge(birthdate, today, 21));
        System.out.println("Is senior (65+)? " + isMinimumAge(birthdate, today, 65));

        // Age groups
        System.out.println("\nAge group:");
        System.out.println(getAgeGroup(birthdate, today));

        // Time until milestone
        System.out.println("\nMilestone birthdays:");
        int[] milestones = {18, 21, 30, 40, 50};
        int currentAge = (int) ChronoUnit.YEARS.between(birthdate, today);
        
        for (int milestone : milestones) {
            if (milestone > currentAge) {
                LocalDate milestoneDate = birthdate.plusYears(milestone);
                long daysUntil = ChronoUnit.DAYS.between(today, milestoneDate);
                System.out.println(milestone + " years: " + milestoneDate + 
                                 " (in " + daysUntil + " days)");
            }
        }

        // Retirement calculation
        System.out.println("\nRetirement:");
        int retirementAge = 65;
        LocalDate retirementDate = birthdate.plusYears(retirementAge);
        long yearsToRetirement = ChronoUnit.YEARS.between(today, retirementDate);
        long daysToRetirement = ChronoUnit.DAYS.between(today, retirementDate);
        System.out.println("Retirement date: " + retirementDate);
        System.out.println("Years to retirement: " + yearsToRetirement);
        System.out.println("Days to retirement: " + daysToRetirement);
    }

    public static boolean isAdult(LocalDate birthdate, LocalDate currentDate) {
        return ChronoUnit.YEARS.between(birthdate, currentDate) >= 18;
    }

    public static boolean isMinimumAge(LocalDate birthdate, LocalDate currentDate, int minAge) {
        return ChronoUnit.YEARS.between(birthdate, currentDate) >= minAge;
    }

    public static String getAgeGroup(LocalDate birthdate, LocalDate currentDate) {
        long age = ChronoUnit.YEARS.between(birthdate, currentDate);
        if (age < 13) return "Child";
        if (age < 18) return "Teenager";
        if (age < 65) return "Adult";
        return "Senior";
    }

}

Day of Week

Dayofweek.java

// Day of week operations

import java.time.LocalDate;
import java.time.DayOfWeek;
import java.time.temporal.TemporalAdjusters;
import java.time.temporal.ChronoUnit;

public class Dayofweek {

    public static void main(String[] args) {
        LocalDate date = ;
        System.out.println("Date: " + date);
        System.out.println();

        // Get day of week
        DayOfWeek dow = date.getDayOfWeek();
        System.out.println("Day of week: " + dow);
        System.out.println("Day value (1=Mon, 7=Sun): " + dow.getValue());

        // Check specific days
        System.out.println("\nCheck specific days:");
        System.out.println("Is Monday? " + (dow == DayOfWeek.MONDAY));
        System.out.println("Is weekend? " + (dow == DayOfWeek.SATURDAY || dow == DayOfWeek.SUNDAY));

        // Next/previous specific day
        System.out.println("\nNext/previous specific day:");
        LocalDate nextMonday = date.with(TemporalAdjusters.next(DayOfWeek.MONDAY));
        System.out.println("Next Monday: " + nextMonday);
        
        LocalDate prevFriday = date.with(TemporalAdjusters.previous(DayOfWeek.FRIDAY));
        System.out.println("Previous Friday: " + prevFriday);

        // Next or same
        LocalDate nextOrSameMon = date.with(TemporalAdjusters.nextOrSame(DayOfWeek.MONDAY));
        System.out.println("Next or same Monday: " + nextOrSameMon);

        // Days in week
        System.out.println("\nAll days of week:");
        for (DayOfWeek day : DayOfWeek.values()) {
            System.out.println(day.getValue() + ": " + day);
        }

        // Week navigation
        System.out.println("\nWeek navigation:");
        LocalDate monday = date.with(TemporalAdjusters.previousOrSame(DayOfWeek.MONDAY));
        System.out.println("This week:");
        for (int i = 0; i < 7; i++) {
            LocalDate d = monday.plusDays(i);
            System.out.println(d.getDayOfWeek() + ": " + d);
        }

        // Business days
        System.out.println("\nBusiness days:");
        System.out.println("Next business day: " + nextBusinessDay(date));
        System.out.println("Previous business day: " + previousBusinessDay(date));

        // Count business days
        LocalDate start = LocalDate.of(2025, 1, 27);  // Monday
        LocalDate end = LocalDate.of(2025, 2, 7);    // Friday
        System.out.println("\nBusiness days from " + start + " to " + end + ": " + 
                          countBusinessDays(start, end));

        // Weekend check
        System.out.println("\nWeekend check:");
        LocalDate[] testDates = {
            LocalDate.of(2025, 1, 27),  // Monday
            LocalDate.of(2025, 2, 1),   // Saturday
            LocalDate.of(2025, 2, 2)    // Sunday
        };
        
        for (LocalDate d : testDates) {
            System.out.println(d + " (" + d.getDayOfWeek() + "): " + 
                             (isWeekend(d) ? "Weekend" : "Weekday"));
        }

        // Add business days
        System.out.println("\nAdd business days:");
        LocalDate today = LocalDate.of(2025, 1, 29);  // Wednesday
        int businessDaysToAdd = ;
        System.out.println("Today: " + today + " (" + today.getDayOfWeek() + ")");
        System.out.println("+" + businessDaysToAdd + " business days: " + addBusinessDays(today, businessDaysToAdd));
    }

    public static LocalDate nextBusinessDay(LocalDate date) {
        LocalDate next = date.plusDays(1);
        while (isWeekend(next)) {
            next = next.plusDays(1);
        }
        return next;
    }

    public static LocalDate previousBusinessDay(LocalDate date) {
        LocalDate prev = date.minusDays(1);
        while (isWeekend(prev)) {
            prev = prev.minusDays(1);
        }
        return prev;
    }

    public static boolean isWeekend(LocalDate date) {
        DayOfWeek dow = date.getDayOfWeek();
        return dow == DayOfWeek.SATURDAY || dow == DayOfWeek.SUNDAY;
    }

    public static long countBusinessDays(LocalDate start, LocalDate end) {
        long count = 0;
        LocalDate current = start;
        while (!current.isAfter(end)) {
            if (!isWeekend(current)) {
                count++;
            }
            current = current.plusDays(1);
        }
        return count;
    }

    public static LocalDate addBusinessDays(LocalDate date, int days) {
        LocalDate result = date;
        int added = 0;
        while (added < days) {
            result = result.plusDays(1);
            if (!isWeekend(result)) {
                added++;
            }
        }
        return result;
    }

}

// Day of week operations

import java.time.LocalDate;
import java.time.DayOfWeek;
import java.time.temporal.TemporalAdjusters;
import java.time.temporal.ChronoUnit;

public class Dayofweek {

    public static void main(String[] args) {
        LocalDate date = ;
        System.out.println("Date: " + date);
        System.out.println();

        // Get day of week
        DayOfWeek dow = date.getDayOfWeek();
        System.out.println("Day of week: " + dow);
        System.out.println("Day value (1=Mon, 7=Sun): " + dow.getValue());

        // Check specific days
        System.out.println("\nCheck specific days:");
        System.out.println("Is Monday? " + (dow == DayOfWeek.MONDAY));
        System.out.println("Is weekend? " + (dow == DayOfWeek.SATURDAY || dow == DayOfWeek.SUNDAY));

        // Next/previous specific day
        System.out.println("\nNext/previous specific day:");
        LocalDate nextMonday = date.with(TemporalAdjusters.next(DayOfWeek.MONDAY));
        System.out.println("Next Monday: " + nextMonday);
        
        LocalDate prevFriday = date.with(TemporalAdjusters.previous(DayOfWeek.FRIDAY));
        System.out.println("Previous Friday: " + prevFriday);

        // Next or same
        LocalDate nextOrSameMon = date.with(TemporalAdjusters.nextOrSame(DayOfWeek.MONDAY));
        System.out.println("Next or same Monday: " + nextOrSameMon);

        // Days in week
        System.out.println("\nAll days of week:");
        for (DayOfWeek day : DayOfWeek.values()) {
            System.out.println(day.getValue() + ": " + day);
        }

        // Week navigation
        System.out.println("\nWeek navigation:");
        LocalDate monday = date.with(TemporalAdjusters.previousOrSame(DayOfWeek.MONDAY));
        System.out.println("This week:");
        for (int i = 0; i < 7; i++) {
            LocalDate d = monday.plusDays(i);
            System.out.println(d.getDayOfWeek() + ": " + d);
        }

        // Business days
        System.out.println("\nBusiness days:");
        System.out.println("Next business day: " + nextBusinessDay(date));
        System.out.println("Previous business day: " + previousBusinessDay(date));

        // Count business days
        LocalDate start = LocalDate.of(2025, 1, 27);  // Monday
        LocalDate end = LocalDate.of(2025, 2, 7);    // Friday
        System.out.println("\nBusiness days from " + start + " to " + end + ": " + 
                          countBusinessDays(start, end));

        // Weekend check
        System.out.println("\nWeekend check:");
        LocalDate[] testDates = {
            LocalDate.of(2025, 1, 27),  // Monday
            LocalDate.of(2025, 2, 1),   // Saturday
            LocalDate.of(2025, 2, 2)    // Sunday
        };
        
        for (LocalDate d : testDates) {
            System.out.println(d + " (" + d.getDayOfWeek() + "): " + 
                             (isWeekend(d) ? "Weekend" : "Weekday"));
        }

        // Add business days
        System.out.println("\nAdd business days:");
        LocalDate today = LocalDate.of(2025, 1, 29);  // Wednesday
        int businessDaysToAdd = ;
        System.out.println("Today: " + today + " (" + today.getDayOfWeek() + ")");
        System.out.println("+" + businessDaysToAdd + " business days: " + addBusinessDays(today, businessDaysToAdd));
    }

    public static LocalDate nextBusinessDay(LocalDate date) {
        LocalDate next = date.plusDays(1);
        while (isWeekend(next)) {
            next = next.plusDays(1);
        }
        return next;
    }

    public static LocalDate previousBusinessDay(LocalDate date) {
        LocalDate prev = date.minusDays(1);
        while (isWeekend(prev)) {
            prev = prev.minusDays(1);
        }
        return prev;
    }

    public static boolean isWeekend(LocalDate date) {
        DayOfWeek dow = date.getDayOfWeek();
        return dow == DayOfWeek.SATURDAY || dow == DayOfWeek.SUNDAY;
    }

    public static long countBusinessDays(LocalDate start, LocalDate end) {
        long count = 0;
        LocalDate current = start;
        while (!current.isAfter(end)) {
            if (!isWeekend(current)) {
                count++;
            }
            current = current.plusDays(1);
        }
        return count;
    }

    public static LocalDate addBusinessDays(LocalDate date, int days) {
        LocalDate result = date;
        int added = 0;
        while (added < days) {
            result = result.plusDays(1);
            if (!isWeekend(result)) {
                added++;
            }
        }
        return result;
    }

}

// Day of week operations

import java.time.LocalDate;
import java.time.DayOfWeek;
import java.time.temporal.TemporalAdjusters;
import java.time.temporal.ChronoUnit;

public class Dayofweek {

    public static void main(String[] args) {
        LocalDate date = ;
        System.out.println("Date: " + date);
        System.out.println();

        // Get day of week
        DayOfWeek dow = date.getDayOfWeek();
        System.out.println("Day of week: " + dow);
        System.out.println("Day value (1=Mon, 7=Sun): " + dow.getValue());

        // Check specific days
        System.out.println("\nCheck specific days:");
        System.out.println("Is Monday? " + (dow == DayOfWeek.MONDAY));
        System.out.println("Is weekend? " + (dow == DayOfWeek.SATURDAY || dow == DayOfWeek.SUNDAY));

        // Next/previous specific day
        System.out.println("\nNext/previous specific day:");
        LocalDate nextMonday = date.with(TemporalAdjusters.next(DayOfWeek.MONDAY));
        System.out.println("Next Monday: " + nextMonday);
        
        LocalDate prevFriday = date.with(TemporalAdjusters.previous(DayOfWeek.FRIDAY));
        System.out.println("Previous Friday: " + prevFriday);

        // Next or same
        LocalDate nextOrSameMon = date.with(TemporalAdjusters.nextOrSame(DayOfWeek.MONDAY));
        System.out.println("Next or same Monday: " + nextOrSameMon);

        // Days in week
        System.out.println("\nAll days of week:");
        for (DayOfWeek day : DayOfWeek.values()) {
            System.out.println(day.getValue() + ": " + day);
        }

        // Week navigation
        System.out.println("\nWeek navigation:");
        LocalDate monday = date.with(TemporalAdjusters.previousOrSame(DayOfWeek.MONDAY));
        System.out.println("This week:");
        for (int i = 0; i < 7; i++) {
            LocalDate d = monday.plusDays(i);
            System.out.println(d.getDayOfWeek() + ": " + d);
        }

        // Business days
        System.out.println("\nBusiness days:");
        System.out.println("Next business day: " + nextBusinessDay(date));
        System.out.println("Previous business day: " + previousBusinessDay(date));

        // Count business days
        LocalDate start = LocalDate.of(2025, 1, 27);  // Monday
        LocalDate end = LocalDate.of(2025, 2, 7);    // Friday
        System.out.println("\nBusiness days from " + start + " to " + end + ": " + 
                          countBusinessDays(start, end));

        // Weekend check
        System.out.println("\nWeekend check:");
        LocalDate[] testDates = {
            LocalDate.of(2025, 1, 27),  // Monday
            LocalDate.of(2025, 2, 1),   // Saturday
            LocalDate.of(2025, 2, 2)    // Sunday
        };
        
        for (LocalDate d : testDates) {
            System.out.println(d + " (" + d.getDayOfWeek() + "): " + 
                             (isWeekend(d) ? "Weekend" : "Weekday"));
        }

        // Add business days
        System.out.println("\nAdd business days:");
        LocalDate today = LocalDate.of(2025, 1, 29);  // Wednesday
        int businessDaysToAdd = ;
        System.out.println("Today: " + today + " (" + today.getDayOfWeek() + ")");
        System.out.println("+" + businessDaysToAdd + " business days: " + addBusinessDays(today, businessDaysToAdd));
    }

    public static LocalDate nextBusinessDay(LocalDate date) {
        LocalDate next = date.plusDays(1);
        while (isWeekend(next)) {
            next = next.plusDays(1);
        }
        return next;
    }

    public static LocalDate previousBusinessDay(LocalDate date) {
        LocalDate prev = date.minusDays(1);
        while (isWeekend(prev)) {
            prev = prev.minusDays(1);
        }
        return prev;
    }

    public static boolean isWeekend(LocalDate date) {
        DayOfWeek dow = date.getDayOfWeek();
        return dow == DayOfWeek.SATURDAY || dow == DayOfWeek.SUNDAY;
    }

    public static long countBusinessDays(LocalDate start, LocalDate end) {
        long count = 0;
        LocalDate current = start;
        while (!current.isAfter(end)) {
            if (!isWeekend(current)) {
                count++;
            }
            current = current.plusDays(1);
        }
        return count;
    }

    public static LocalDate addBusinessDays(LocalDate date, int days) {
        LocalDate result = date;
        int added = 0;
        while (added < days) {
            result = result.plusDays(1);
            if (!isWeekend(result)) {
                added++;
            }
        }
        return result;
    }

}

// Day of week operations

import java.time.LocalDate;
import java.time.DayOfWeek;
import java.time.temporal.TemporalAdjusters;
import java.time.temporal.ChronoUnit;

public class Dayofweek {

    public static void main(String[] args) {
        LocalDate date = ;
        System.out.println("Date: " + date);
        System.out.println();

        // Get day of week
        DayOfWeek dow = date.getDayOfWeek();
        System.out.println("Day of week: " + dow);
        System.out.println("Day value (1=Mon, 7=Sun): " + dow.getValue());

        // Check specific days
        System.out.println("\nCheck specific days:");
        System.out.println("Is Monday? " + (dow == DayOfWeek.MONDAY));
        System.out.println("Is weekend? " + (dow == DayOfWeek.SATURDAY || dow == DayOfWeek.SUNDAY));

        // Next/previous specific day
        System.out.println("\nNext/previous specific day:");
        LocalDate nextMonday = date.with(TemporalAdjusters.next(DayOfWeek.MONDAY));
        System.out.println("Next Monday: " + nextMonday);
        
        LocalDate prevFriday = date.with(TemporalAdjusters.previous(DayOfWeek.FRIDAY));
        System.out.println("Previous Friday: " + prevFriday);

        // Next or same
        LocalDate nextOrSameMon = date.with(TemporalAdjusters.nextOrSame(DayOfWeek.MONDAY));
        System.out.println("Next or same Monday: " + nextOrSameMon);

        // Days in week
        System.out.println("\nAll days of week:");
        for (DayOfWeek day : DayOfWeek.values()) {
            System.out.println(day.getValue() + ": " + day);
        }

        // Week navigation
        System.out.println("\nWeek navigation:");
        LocalDate monday = date.with(TemporalAdjusters.previousOrSame(DayOfWeek.MONDAY));
        System.out.println("This week:");
        for (int i = 0; i < 7; i++) {
            LocalDate d = monday.plusDays(i);
            System.out.println(d.getDayOfWeek() + ": " + d);
        }

        // Business days
        System.out.println("\nBusiness days:");
        System.out.println("Next business day: " + nextBusinessDay(date));
        System.out.println("Previous business day: " + previousBusinessDay(date));

        // Count business days
        LocalDate start = LocalDate.of(2025, 1, 27);  // Monday
        LocalDate end = LocalDate.of(2025, 2, 7);    // Friday
        System.out.println("\nBusiness days from " + start + " to " + end + ": " + 
                          countBusinessDays(start, end));

        // Weekend check
        System.out.println("\nWeekend check:");
        LocalDate[] testDates = {
            LocalDate.of(2025, 1, 27),  // Monday
            LocalDate.of(2025, 2, 1),   // Saturday
            LocalDate.of(2025, 2, 2)    // Sunday
        };
        
        for (LocalDate d : testDates) {
            System.out.println(d + " (" + d.getDayOfWeek() + "): " + 
                             (isWeekend(d) ? "Weekend" : "Weekday"));
        }

        // Add business days
        System.out.println("\nAdd business days:");
        LocalDate today = LocalDate.of(2025, 1, 29);  // Wednesday
        int businessDaysToAdd = ;
        System.out.println("Today: " + today + " (" + today.getDayOfWeek() + ")");
        System.out.println("+" + businessDaysToAdd + " business days: " + addBusinessDays(today, businessDaysToAdd));
    }

    public static LocalDate nextBusinessDay(LocalDate date) {
        LocalDate next = date.plusDays(1);
        while (isWeekend(next)) {
            next = next.plusDays(1);
        }
        return next;
    }

    public static LocalDate previousBusinessDay(LocalDate date) {
        LocalDate prev = date.minusDays(1);
        while (isWeekend(prev)) {
            prev = prev.minusDays(1);
        }
        return prev;
    }

    public static boolean isWeekend(LocalDate date) {
        DayOfWeek dow = date.getDayOfWeek();
        return dow == DayOfWeek.SATURDAY || dow == DayOfWeek.SUNDAY;
    }

    public static long countBusinessDays(LocalDate start, LocalDate end) {
        long count = 0;
        LocalDate current = start;
        while (!current.isAfter(end)) {
            if (!isWeekend(current)) {
                count++;
            }
            current = current.plusDays(1);
        }
        return count;
    }

    public static LocalDate addBusinessDays(LocalDate date, int days) {
        LocalDate result = date;
        int added = 0;
        while (added < days) {
            result = result.plusDays(1);
            if (!isWeekend(result)) {
                added++;
            }
        }
        return result;
    }

}

Month Operations

MonthCalc.java

// Month operations

import java.time.LocalDate;
import java.time.Month;
import java.time.YearMonth;
import java.time.temporal.TemporalAdjusters;

public class MonthCalc {

    public static void main(String[] args) {
        LocalDate date = ;
        System.out.println("Date: " + date);
        System.out.println();

        // Get month
        Month month = date.getMonth();
        System.out.println("Month: " + month);
        System.out.println("Month value: " + month.getValue());
        System.out.println("Month length: " + month.length(date.isLeapYear()));

        // Month navigation
        System.out.println("\nMonth navigation:");
        System.out.println("Next month: " + month.plus(1));
        System.out.println("Previous month: " + month.minus(1));

        // All months
        System.out.println("\nAll months:");
        for (Month m : Month.values()) {
            System.out.println(m.getValue() + ": " + m);
        }

        // First/last day of month
        System.out.println("\nFirst/last day of month:");
        LocalDate firstDay = date.with(TemporalAdjusters.firstDayOfMonth());
        LocalDate lastDay = date.with(TemporalAdjusters.lastDayOfMonth());
        System.out.println("First day: " + firstDay);
        System.out.println("Last day: " + lastDay);

        // YearMonth
        System.out.println("\nYearMonth:");
        YearMonth ym = YearMonth.of(2025, 1);
        System.out.println("YearMonth: " + ym);
        System.out.println("Length: " + ym.lengthOfMonth() + " days");
        System.out.println("Is leap year: " + ym.isLeapYear());
        System.out.println("First day: " + ym.atDay(1));
        System.out.println("Last day: " + ym.atEndOfMonth());

        // Month lengths
        System.out.println("\nMonth lengths (2025):");
        for (Month m : Month.values()) {
            YearMonth yearMonth = YearMonth.of(2025, m);
            System.out.println(m + ": " + yearMonth.lengthOfMonth() + " days");
        }

        // Leap year comparison
        System.out.println("\nFebruary length:");
        System.out.println("2024 (leap): " + YearMonth.of(2024, 2).lengthOfMonth());
        System.out.println("2025 (not): " + YearMonth.of(2025, 2).lengthOfMonth());

        // Month range
        System.out.println("\nMonth range:");
        LocalDate start = LocalDate.of(2025, 1, 15);
        LocalDate end = LocalDate.of(2025, 3, 20);
        System.out.println("From " + start + " to " + end);
        
        YearMonth current = YearMonth.from(start);
        YearMonth last = YearMonth.from(end);
        while (!current.isAfter(last)) {
            System.out.println(current + " (" + current.lengthOfMonth() + " days)");
            current = current.plusMonths(1);
        }

        // Add months with overflow
        System.out.println("\nAdd months (handle overflow):");
        LocalDate jan31 = LocalDate.of(2025, 1, 31);
        System.out.println("Jan 31: " + jan31);
        System.out.println("+1 month: " + jan31.plusMonths(1));  // Feb 28
        System.out.println("+2 months: " + jan31.plusMonths(2)); // Mar 31

        // Practical examples
        System.out.println("\nPractical examples:");
        
        // Quarterly dates
        LocalDate q1End = LocalDate.of(2025, 3, 31);
        LocalDate q2End = q1End.plusMonths(3);
        LocalDate q3End = q2End.plusMonths(3);
        LocalDate q4End = q3End.plusMonths(3);
        System.out.println("Q1 end: " + q1End);
        System.out.println("Q2 end: " + q2End);
        System.out.println("Q3 end: " + q3End);
        System.out.println("Q4 end: " + q4End);

        // Monthly billing dates
        System.out.println("\nMonthly billing dates (starting " + date + "):");
        for (int i = 0; i < 6; i++) {
            LocalDate billing = date.plusMonths(i);
            System.out.println("Month " + i + ": " + billing);
        }

        // End of each month
        System.out.println("\nEnd of each month (2025):");
        for (int m = 1; m <= 12; m++) {
            LocalDate monthEnd = YearMonth.of(2025, m).atEndOfMonth();
            System.out.println(monthEnd.getMonth() + ": " + monthEnd);
        }
    }

}

// Month operations

import java.time.LocalDate;
import java.time.Month;
import java.time.YearMonth;
import java.time.temporal.TemporalAdjusters;

public class MonthCalc {

    public static void main(String[] args) {
        LocalDate date = ;
        System.out.println("Date: " + date);
        System.out.println();

        // Get month
        Month month = date.getMonth();
        System.out.println("Month: " + month);
        System.out.println("Month value: " + month.getValue());
        System.out.println("Month length: " + month.length(date.isLeapYear()));

        // Month navigation
        System.out.println("\nMonth navigation:");
        System.out.println("Next month: " + month.plus(1));
        System.out.println("Previous month: " + month.minus(1));

        // All months
        System.out.println("\nAll months:");
        for (Month m : Month.values()) {
            System.out.println(m.getValue() + ": " + m);
        }

        // First/last day of month
        System.out.println("\nFirst/last day of month:");
        LocalDate firstDay = date.with(TemporalAdjusters.firstDayOfMonth());
        LocalDate lastDay = date.with(TemporalAdjusters.lastDayOfMonth());
        System.out.println("First day: " + firstDay);
        System.out.println("Last day: " + lastDay);

        // YearMonth
        System.out.println("\nYearMonth:");
        YearMonth ym = YearMonth.of(2025, 1);
        System.out.println("YearMonth: " + ym);
        System.out.println("Length: " + ym.lengthOfMonth() + " days");
        System.out.println("Is leap year: " + ym.isLeapYear());
        System.out.println("First day: " + ym.atDay(1));
        System.out.println("Last day: " + ym.atEndOfMonth());

        // Month lengths
        System.out.println("\nMonth lengths (2025):");
        for (Month m : Month.values()) {
            YearMonth yearMonth = YearMonth.of(2025, m);
            System.out.println(m + ": " + yearMonth.lengthOfMonth() + " days");
        }

        // Leap year comparison
        System.out.println("\nFebruary length:");
        System.out.println("2024 (leap): " + YearMonth.of(2024, 2).lengthOfMonth());
        System.out.println("2025 (not): " + YearMonth.of(2025, 2).lengthOfMonth());

        // Month range
        System.out.println("\nMonth range:");
        LocalDate start = LocalDate.of(2025, 1, 15);
        LocalDate end = LocalDate.of(2025, 3, 20);
        System.out.println("From " + start + " to " + end);
        
        YearMonth current = YearMonth.from(start);
        YearMonth last = YearMonth.from(end);
        while (!current.isAfter(last)) {
            System.out.println(current + " (" + current.lengthOfMonth() + " days)");
            current = current.plusMonths(1);
        }

        // Add months with overflow
        System.out.println("\nAdd months (handle overflow):");
        LocalDate jan31 = LocalDate.of(2025, 1, 31);
        System.out.println("Jan 31: " + jan31);
        System.out.println("+1 month: " + jan31.plusMonths(1));  // Feb 28
        System.out.println("+2 months: " + jan31.plusMonths(2)); // Mar 31

        // Practical examples
        System.out.println("\nPractical examples:");
        
        // Quarterly dates
        LocalDate q1End = LocalDate.of(2025, 3, 31);
        LocalDate q2End = q1End.plusMonths(3);
        LocalDate q3End = q2End.plusMonths(3);
        LocalDate q4End = q3End.plusMonths(3);
        System.out.println("Q1 end: " + q1End);
        System.out.println("Q2 end: " + q2End);
        System.out.println("Q3 end: " + q3End);
        System.out.println("Q4 end: " + q4End);

        // Monthly billing dates
        System.out.println("\nMonthly billing dates (starting " + date + "):");
        for (int i = 0; i < 6; i++) {
            LocalDate billing = date.plusMonths(i);
            System.out.println("Month " + i + ": " + billing);
        }

        // End of each month
        System.out.println("\nEnd of each month (2025):");
        for (int m = 1; m <= 12; m++) {
            LocalDate monthEnd = YearMonth.of(2025, m).atEndOfMonth();
            System.out.println(monthEnd.getMonth() + ": " + monthEnd);
        }
    }

}

TemporalAdjuster A strategy for adjusting dates, like finding the first/last day of month or next specific weekday.

Period vs Duration

Period: Date-based (days, months, years)
Duration: Time-based (hours, minutes, seconds)

Common Operations

Add/subtract days, months, years
Calculate duration between dates
Find day of week, month
First/last day of month

Exercise: Practical.java

Calculate the number of business days (excluding weekends) between two dates