Calculate week of month in .NET

There is no built in way to do this but here is an extension method that should do the job for you:

static class DateTimeExtensions {
    static GregorianCalendar _gc = new GregorianCalendar();
    public static int GetWeekOfMonth(this DateTime time) {
        DateTime first = new DateTime(time.Year, time.Month, 1);
        return time.GetWeekOfYear() - first.GetWeekOfYear() + 1;
    }

    static int GetWeekOfYear(this DateTime time) {
        return _gc.GetWeekOfYear(time, CalendarWeekRule.FirstDay, DayOfWeek.Sunday);
    }
}

Usage:

DateTime time = new DateTime(2010, 1, 25);
Console.WriteLine(time.GetWeekOfMonth());

Output:

5

You can alter GetWeekOfYear according to your needs.

There is no direct built-in way to do this, but it can be done quite easily. Here is an extension method which can be used to easily get the year-based week number of a date:

public static int GetWeekNumber(this DateTime date)
{
    return GetWeekNumber(date, CultureInfo.CurrentCulture);
}

public static int GetWeekNumber(this DateTime date, CultureInfo culture)
{
    return culture.Calendar.GetWeekOfYear(date,
        culture.DateTimeFormat.CalendarWeekRule,
        culture.DateTimeFormat.FirstDayOfWeek);
}

We can then use that to calculate the month-based week number, kind of like Jason shows. A culture friendly version could look something like this:

public static int GetWeekNumberOfMonth(this DateTime date)
{
    return GetWeekNumberOfMonth(date, CultureInfo.CurrentCulture);
}

public static int GetWeekNumberOfMonth(this DateTime date, CultureInfo culture)
{
    return date.GetWeekNumber(culture)
         - new DateTime(date.Year, date.Month, 1).GetWeekNumber(culture)
         + 1; // Or skip +1 if you want the first week to be 0.
}

As specified here: http://forums.asp.net/t/1268112.aspx

you can use:

public static int Iso8601WeekNumber(DateTime dt)
{
    return  CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(dt, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
}

public static int GetWeekOfMonth(DateTime dt)
{    
        
    int weekOfYear = Iso8601WeekNumber(dt);
    int weekOfYearAtFirst = Iso8601WeekNumber(dt.AddDays(1 - dt.Day));
    
    if (dt.Month == 1)
    {
        //week of year == week of month in January unless
        //December wraps
        if (weekOfYearAtFirst > 6) 
        {
            return weekOfYear > 6 ? 1 : weekOfYear + 1;
        }
        else 
        {
            return weekOfYear;
        }
    }

    return weekOfYear - weekOfYearAtFirst + 1;
}

Note thought that your needs may vary as there are certain algorithmic choices. I.e. is a week with Monday-Friday at the end of February also the first week of March, or what criteria defines the "first" week of a month? ISO 8601:2004 does not give a definition for "Week of Month".

This is an old thread but I needed something like this and this is what I come up with.

public static int WeekOfMonth(this DateTime date)
{
    DateTime firstDayOfMonth = new DateTime(date.Year, date.Month, 1);
    int firstDay = (int)firstDayOfMonth.DayOfWeek;
    if (firstDay == 0)
    {
        firstDay = 7;
    }
    double d = (firstDay + date.Day - 1) / 7.0;
    return (int)Math.Ceiling(d);
}

Note: This should work if you want to use Monday as the first day of the week. If you want the week to start on Sunday, this fails in 2nd week of March, 2015 and probably other dates. Monday March 9th, 2015 is the first day of the second week. The calculation above returns "3" for this date.

Why not as simple as this extension?

public static int WeekNumber( this DateTime d )
{
    int w = 0;

    if ( d.Day <= 7 )
    {
        w = 1;
    }
    else if ( d.Day > 7 && d.Day <= 14 )
    {
        w = 2;
    }
    else if ( d.Day > 14 && d.Day <= 21 )
    {
        w = 3;
    }
    else if ( d.Day > 21 && d.Day <= 28 )
    {
        w = 4;
    }
    else if ( d.Day > 28 )
    {
        w = 5;
    }

    return w;
}