Correctly Getting The Number Of Days In A Month In C# & .NET

Our last post, “Getting The Number Of Days In A Month In C# & .NET”, looked at two ways to get the number of days in a particular month.

The code is correct, but it may potentially be wrong..

Why?

Calendar systems!

The calendar most of the world currently uses is the Gregorian Calendar.

The proper way to get the days in the month in this calendar is to use the GregorianCalendar class as follows:

var dateOnly = new DateOnly(2000, 1, 1);
var gregorian = new GregorianCalendar();
var daysInMonth = gregorian.GetDaysInMonth(dateOnly.Year, dateOnly.Month);
Console.WriteLine($"{dateOnly:MMMM yyyy} has {daysInMonth} days in the Gregorian Calendar");

This will print the following:

January 2000 has 31 days in the Gregorian Calendar

If you were using another calendar, say Hijri, you would use the HijriCalendar class as follows:

var dateOnly = new DateOnly(2000, 1, 1);
var hijri = new HijriCalendar();
var daysInMonth = hijri.GetDaysInMonth(dateOnly.Year, dateOnly.Month);
Console.WriteLine($"{dateOnly:MMMM yyyy} has {daysInMonth} days in the Hijri Calendar");

This will print the following:

January 2000 has 30 days in the Hijri Calendar

We can pull a list of calendars using reflection as folllows:

var calendar = typeof(Calendar);
// Get the assembly
var calendars = calendar.Assembly
    // Get the types in the assembly
    .GetTypes()
    // Filter out the non-abstract classes, and get those that are assignable
    .Where(t => !t.IsAbstract && calendar.IsAssignableFrom(t))
    // Order by name
    .OrderBy(t => t.Name);

Console.WriteLine("Calendars:");

foreach (var type in calendars!)
{
    Console.WriteLine($"- {type.Name}");
}

This code will print the following:

Calendars:
- ChineseLunisolarCalendar
- GregorianCalendar
- HebrewCalendar
- HijriCalendar
- JapaneseCalendar
- JapaneseLunisolarCalendar
- JulianCalendar
- KoreanCalendar
- KoreanLunisolarCalendar
- PersianCalendar
- TaiwanCalendar
- TaiwanLunisolarCalendar
- ThaiBuddhistCalendar
- UmAlQuraCalendar

These are the calendars currently supported by .NET.

TLDR

Days in the month are a construct of a calendar, as well as the year and the month.

The code is in my GitHub.

Happy hacking!