Dates, Times, and Formatting

Date Basics

Create a fixed date and read its parts.

dates A `DateTime` can represent a calendar date with year, month, and day parts.

Date Basics

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

class Program
{
    static void Main()
    {
        int day = 15;
        DateTime lessonDate = new DateTime(2026, 3, day);
        string isoDate = lessonDate.ToString("yyyy-MM-dd", CultureInfo.InvariantCulture);
        string month = lessonDate.Month.ToString(CultureInfo.InvariantCulture);

        Console.WriteLine($"date={isoDate}");
        Console.WriteLine($"month={month}");
    }
}

using System;
using System.Globalization;

class Program
{
    static void Main()
    {
        int day = 1;
        DateTime lessonDate = new DateTime(2026, 3, day);
        string isoDate = lessonDate.ToString("yyyy-MM-dd", CultureInfo.InvariantCulture);
        string month = lessonDate.Month.ToString(CultureInfo.InvariantCulture);

        Console.WriteLine($"date={isoDate}");
        Console.WriteLine($"month={month}");
    }
}

using System;
using System.Globalization;

class Program
{
    static void Main()
    {
        int day = 28;
        DateTime lessonDate = new DateTime(2026, 3, day);
        string isoDate = lessonDate.ToString("yyyy-MM-dd", CultureInfo.InvariantCulture);
        string month = lessonDate.Month.ToString(CultureInfo.InvariantCulture);

        Console.WriteLine($"date={isoDate}");
        Console.WriteLine($"month={month}");
    }
}

  1. day ← 15, lessonDate ← 03/15/2026 00:00:00, isoDate ← 2026-03-15

    5{6    static void Main()7    {8        int day→ 15 = 15; //@day=1, 289        DateTime lessonDate→ 03/15/2026 00:00:00 = new DateTime(2026, 3, day);10        string isoDate→ 2026-03-15 = lessonDate03/15/2026 00:00:00.ToString("yyyy-MM-dd", CultureInfo.InvariantCulture);11        string month→ 3 = lessonDate.Month3.ToString(CultureInfo.InvariantCulture);1213        Console.WriteLine($"date={isoDate2026-03-15}");14        Console.WriteLine($"month={month3}");15    }
    outputdate=2026-03-15
month=3

  1. day ← 1, lessonDate ← 03/01/2026 00:00:00, isoDate ← 2026-03-01

    5{6    static void Main()7    {8        int day→ 1 = 1;9        DateTime lessonDate→ 03/01/2026 00:00:00 = new DateTime(2026, 3, day);10        string isoDate→ 2026-03-01 = lessonDate03/01/2026 00:00:00.ToString("yyyy-MM-dd", CultureInfo.InvariantCulture);11        string month→ 3 = lessonDate.Month3.ToString(CultureInfo.InvariantCulture);1213        Console.WriteLine($"date={isoDate2026-03-01}");14        Console.WriteLine($"month={month3}");15    }
    outputdate=2026-03-01
month=3

  1. day ← 28, lessonDate ← 03/28/2026 00:00:00, isoDate ← 2026-03-28

    5{6    static void Main()7    {8        int day→ 28 = 28;9        DateTime lessonDate→ 03/28/2026 00:00:00 = new DateTime(2026, 3, day);10        string isoDate→ 2026-03-28 = lessonDate03/28/2026 00:00:00.ToString("yyyy-MM-dd", CultureInfo.InvariantCulture);11        string month→ 3 = lessonDate.Month3.ToString(CultureInfo.InvariantCulture);1213        Console.WriteLine($"date={isoDate2026-03-28}");14        Console.WriteLine($"month={month3}");15    }
    outputdate=2026-03-28
month=3