Mark Ashley Bell

02 Feb 2019

Getting the first and last day of a week or month with C#

I recently found myself needing to get the date of the first day of a particular week in C#. After browsing this rather confusing collection of answers, I pieced together parts of various code snippets into this little collection of utility methods.

public static partial class DateTimeExtensions 
    public static DateTime FirstDayOfWeek(this DateTime dt)
        var culture = System.Threading.Thread.CurrentThread.CurrentCulture;
        var diff = dt.DayOfWeek - culture.DateTimeFormat.FirstDayOfWeek;

        if (diff < 0)
            diff += 7;

        return dt.AddDays(-diff).Date;

    public static DateTime LastDayOfWeek(this DateTime dt) =>

    public static DateTime FirstDayOfMonth(this DateTime dt) =>
        new DateTime(dt.Year, dt.Month, 1);

    public static DateTime LastDayOfMonth(this DateTime dt) =>

    public static DateTime FirstDayOfNextMonth(this DateTime dt) =>

The FirstDayOfWeek method is culture-sensitive, so in the en-GB culture the first day will be a Monday, whereas in the en-US culture it will be a Sunday. Given that, you can now easily get month and week boundaries for any given DateTime:

var firstdayOfThisWeek = DateTime.Now.FirstDayOfWeek();

Hopefully someone else out there will find these useful!

Originally published on 02 Feb 2015; updated on 02 Feb 2019

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