Dates, Times, and Formatting

Date Arithmetic

Add days to a fixed date and compare the result.

dates Date methods return new values, so arithmetic can be assigned and formatted.

Date Arithmetic

addDays
DateArithmetic.cs
Replay: real traced execution (multi-file project)
using System;
using System.Globalization;

class Program
{
    static void Main()
    {
        int addDays = 3;
        DateTime start = new DateTime(2026, 4, 10);
        DateTime due = start.AddDays(addDays);
        bool sameMonth = start.Month == due.Month;

        string dueText = due.ToString("yyyy-MM-dd", CultureInfo.InvariantCulture);

        Console.WriteLine($"due={dueText}");
        Console.WriteLine($"sameMonth={sameMonth}");
    }
}

using System;
using System.Globalization;

class Program
{
    static void Main()
    {
        int addDays = 1;
        DateTime start = new DateTime(2026, 4, 10);
        DateTime due = start.AddDays(addDays);
        bool sameMonth = start.Month == due.Month;

        string dueText = due.ToString("yyyy-MM-dd", CultureInfo.InvariantCulture);

        Console.WriteLine($"due={dueText}");
        Console.WriteLine($"sameMonth={sameMonth}");
    }
}

using System;
using System.Globalization;

class Program
{
    static void Main()
    {
        int addDays = 7;
        DateTime start = new DateTime(2026, 4, 10);
        DateTime due = start.AddDays(addDays);
        bool sameMonth = start.Month == due.Month;

        string dueText = due.ToString("yyyy-MM-dd", CultureInfo.InvariantCulture);

        Console.WriteLine($"due={dueText}");
        Console.WriteLine($"sameMonth={sameMonth}");
    }
}

  1. addDays ← 3, start ← 04/10/2026 00:00:00, due ← 04/13/2026 00:00:00

    5{6    static void Main()7    {8        int addDays→ 3 = 3; //@addDays=1, 79        DateTime start→ 04/10/2026 00:00:00 = new DateTime(2026, 4, 10);10        DateTime due→ 04/13/2026 00:00:00 = start04/10/2026 00:00:00.AddDays(addDays3);11        bool sameMonth→ True = start.Month4 == due.Month4;1213        string dueText→ 2026-04-13 = due04/13/2026 00:00:00.ToString("yyyy-MM-dd", CultureInfo.InvariantCulture);1415        Console.WriteLine($"due={dueText2026-04-13}");16        Console.WriteLine($"sameMonth={sameMonthTrue}");17    }
    outputdue=2026-04-13
sameMonth=True

  1. addDays ← 1, start ← 04/10/2026 00:00:00, due ← 04/11/2026 00:00:00

    5{6    static void Main()7    {8        int addDays→ 1 = 1;9        DateTime start→ 04/10/2026 00:00:00 = new DateTime(2026, 4, 10);10        DateTime due→ 04/11/2026 00:00:00 = start04/10/2026 00:00:00.AddDays(addDays1);11        bool sameMonth→ True = start.Month4 == due.Month4;1213        string dueText→ 2026-04-11 = due04/11/2026 00:00:00.ToString("yyyy-MM-dd", CultureInfo.InvariantCulture);1415        Console.WriteLine($"due={dueText2026-04-11}");16        Console.WriteLine($"sameMonth={sameMonthTrue}");17    }
    outputdue=2026-04-11
sameMonth=True

  1. addDays ← 7, start ← 04/10/2026 00:00:00, due ← 04/17/2026 00:00:00

    5{6    static void Main()7    {8        int addDays→ 7 = 7;9        DateTime start→ 04/10/2026 00:00:00 = new DateTime(2026, 4, 10);10        DateTime due→ 04/17/2026 00:00:00 = start04/10/2026 00:00:00.AddDays(addDays7);11        bool sameMonth→ True = start.Month4 == due.Month4;1213        string dueText→ 2026-04-17 = due04/17/2026 00:00:00.ToString("yyyy-MM-dd", CultureInfo.InvariantCulture);1415        Console.WriteLine($"due={dueText2026-04-17}");16        Console.WriteLine($"sameMonth={sameMonthTrue}");17    }
    outputdue=2026-04-17
sameMonth=True