Calculate week of month in .NET

Question

Do the .NET libraries have an easy way of returning the week number for a given date? For example, input of Year = 2010, Month = 1, Day = 25, should output 5 for the week number.

Closest I found was Calendar.GetWeekOfYear, which is almost there.

Java has a date string format "W" which returns week in month but I can't see anything equivalent in .NET.

jason · Accepted Answer · 2010-01-26T12:51:36.547

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:

You can alter GetWeekOfYear according to your needs.

score 24 · Answer 2 · edited May 23 '17 at 12:02

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.
}

@eddiegroves: Thank you. At least someone appreciates it :p – Svish Jan 26 '10 at 21:49 — Svish, Jan 26 '10 at 21:49

Jason Gilley · Answer 3 · 2013-08-31T22:13:17.360

14

The title is misleading. The question is really about the calendar week number (the week number if you counted rows on a calendar) of the month, not the actual week number.

For the week number of the month, you can use:

VB:
    Function WeekNumber(TargetDate As Date) As Integer
        Return (TargetDate.Day - 1) \ 7 + 1
    End Function

C#:
    public int WeekNumber(DateTime TargetDate)
    {
        return (TargetDate.Day - 1) / 7 + 1;
    }

edited Aug 31 '13 at 22:13

answered Aug 31 '13 at 22:08

Jason Gilley

193
1
4

2

I agree that the title is misleading. I was trying to calculate, for example, the fourth Monday of the month. However, when I wrote my unit tests using other answers provided under this question, some of the results did not match what I expected. This is because if the first day of the month is Tuesday or later in the week, the results are off by one week. The code provided by this answer is what I ended up using to solve the problem. – user3308241 Jan 23 '15 at 05:35
4

This doesn't line up with the example. Jan 1st of 2010 was Friday, so the first calendar week had only two days. As a result, the 25th, which would normally fall under week 4 using this formula, is actually on week 5. Since the example says that Jan 1st, 2010 should produce week 5, the formula in this answer cannot be right, because it produces week 4 (i.e. (int((25 - 1) / 7) + 1) = 4. – Triynko Feb 18 '16 at 04:31

GaussZ · Answer 4 · 2020-12-13T08:33:01.330

5

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".

edited Dec 13 '20 at 08:33

answered Jan 25 '10 at 23:38

GaussZ

838
10
25

1

This is incorrect; it returns `-48` for `dt = new DateTime(2010, 1, 25)`. – jason Jan 26 '10 at 00:21
@Jason Good catch. Problem was with the first day of January belonging to the last week of December. Fixed the code to ignore that. – GaussZ Jan 26 '10 at 07:53
1

Returns 53 in January 1, 2021 – jaysonragasa Dec 12 '20 at 21:33
@jaysonragasa Yep, that January week belonging to December was not handled correctly. I fixed it in code. – GaussZ Dec 13 '20 at 08:35
@GaussZ > When I change from FirstFourDayWeek to FirstDay, the problem seems to solved. Or it will have any issues? I actually made a dynamic calendar for Xamarin – jaysonragasa Dec 14 '20 at 09:26

score 4 · Answer 5 · answered Nov 02 '15 at 17:56

Here's a simple option if you want the first 7 days to be considered the first week, even if the month doesn't start on a Sunday:

class Program
{
    static void Main(string[] args)
    {
        var dates = Enumerable.Range(0, 10)
            .Select(r => new DateTime(2015, 10, 1).AddDays(r))
            .Select(q => new { Date = q, WeekOfMonth = q.WeekOfMonth() });

        foreach(var item in dates)
        {
            Console.WriteLine("Date: {0:ddd MM-dd-yyyy}, WOM: {1}", item.Date, item.WeekOfMonth);
        }

        var final = Console.ReadLine();
    }
}

public static class DateTimeExtensions
{
    public static int WeekOfMonth(this DateTime d)
    {
        var isExact = (d.Day % 7 == 0);
        var offset = isExact ? 0 : 1;
        return (int)Math.Floor(d.Day / 7.0) + offset;
    }
}

score 3 · Answer 6 · answered Dec 04 '12 at 11:15

@Svish

VB version:

Public Shared Function GetWeekNumber(ByVal [date] As DateTime) As Integer
    Return GetWeekNumber([date], CultureInfo.CurrentCulture)
End Function

Public Shared Function GetWeekNumber(ByVal [date] As DateTime, ByVal culture As CultureInfo) As Integer
    Return culture.Calendar.GetWeekOfYear([date], culture.DateTimeFormat.CalendarWeekRule, culture.DateTimeFormat.FirstDayOfWeek)
End Function

Public Shared Function GetWeekNumberOfMonth(ByVal [date] As DateTime) As Integer
    Return GetWeekNumberOfMonth([date], CultureInfo.CurrentCulture)
End Function

Public Shared Function GetWeekNumberOfMonth(ByVal [date] As DateTime, ByVal culture As CultureInfo) As Integer
    Return GetWeekNumber([date], culture) - GetWeekNumber(New DateTime([date].Year, [date].Month, 1), culture) + 1
    ' Or skip +1 if you want the first week to be 0.
End Function

score 3 · Answer 7 · edited Jun 27 '14 at 12:01

This is my solution:

    static string GetWeekNumber()
    {

        var weekNumber = CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(DateTime.Now, CalendarWeekRule.FirstFourDayWeek,
            DayOfWeek.Sunday) -
        CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(DateTime.Now.AddDays(1 - DateTime.Now.Day),
            CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Sunday) + 1;

        switch (weekNumber)
        {
            case 1 :
                return "First";
            case 2:
                return "Second";
            case 3:
                return "Third";
            default:
                return "Fourth";
        }
    }

Do you know that it is possible of 6 weeks a month? – Igor Golodnitsky Jun 09 '16 at 21:09 — Igor Golodnitsky, Jun 09 '16 at 21:09

score 3 · Answer 8 · edited Nov 21 '12 at 23:14

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.

cazavientos · Answer 9 · 2013-03-12T23:28:30.003

Once you have the DayOfWeek of the first day of month, you can get the week of month using integer arithmetic.

static class DateTimeExtensions
{
    // Assumes that in current culture week starts on Sunday
    public static int GetWeekOfMonth(this DateTimeOffset time)
    {
        DateTimeOffset first = new DateTimeOffset(time.Year, time.Month, 1, 0, 0, 0, time.Offset);
        int firstSunday = (7 - (int)first.DayOfWeek) % 7 + 1;
        int weekOfMonth = 1 + (time.Day + 7 - firstSunday) / 7;

        return weekOfMonth;
    }
}

score 2 · Answer 10 · answered Mar 19 '20 at 03:51

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;
}

score 1 · Answer 11 · edited Jul 14 '12 at 11:18

1

public int WeekCalc(DateTime dt)
{
    CultureInfo ciCurr = CultureInfo.CurrentCulture;
    int weekNum = ciCurr.Calendar.GetWeekOfYear(dt, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
    return weekNum;
}

edited Jul 14 '12 at 11:18

akjoshi

15,374
13
103
121

answered Jul 12 '12 at 07:24

Misi

748
5
21
46

score 1 · Answer 12 · answered Mar 22 '13 at 13:13

Something simple... :)

private int WeeksNumber()
        {
            int daysInMonth = DateTime.DaysInMonth(YourYear, YourMonth);
            DateTime date = new DateTime(Year, Month, 1);
            switch (daysInMonth)
            {
                case 28:
                    {
                        if (date.DayOfWeek == DayOfWeek.Monday)
                        {
                            return 4;
                        }
                        else
                        {
                            return 5;
                        }
                    }
                case 29:
                    {
                        return 5;
                    }
                case 30:
                    {
                        if (date.DayOfWeek == DayOfWeek.Sunday)
                        {
                            return 6;
                        }
                        else
                        {
                            return 5;
                        }
                    }
                case 31:
                    {
                        if (date.DayOfWeek == DayOfWeek.Sunday || date.DayOfWeek == DayOfWeek.Saturday)
                        {
                            return 6;
                        }
                        else
                        {
                            return 5;
                        }
                    }
                default: return 5;
            }
        }

score 1 · Answer 13 · answered Jun 22 '16 at 08:05

  Dim mdt As Date = txtDT.Text
    Dim weekNum As Integer = CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(mdt, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Sunday) - CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(GetFirstDayOfMonth(mdt), CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Sunday) + 1
    Response.Write(weekNum)

Calculate week of month in .NET

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