How can I calculate/find the week-number of a given date?

Question

Jonas Elfström · Accepted Answer · 2016-04-20T09:13:07.650

var currentCulture = CultureInfo.CurrentCulture;
var weekNo = currentCulture.Calendar.GetWeekOfYear(
                new DateTime(2013, 12, 31),
                currentCulture.DateTimeFormat.CalendarWeekRule,
                currentCulture.DateTimeFormat.FirstDayOfWeek);

Be aware that this is not ISO 8601 compatible. In Sweden we use ISO 8601 week numbers but even though the culture is set to "sv-SE", CalendarWeekRule is FirstFourDayWeek, and FirstDayOfWeek is Monday the weekNo variable will be set to 53 instead of the correct 1 in the above code.

I have only tried this with Swedish settings but I'm pretty sure that all countries (Austria, Germany, Switzerland and more) using ISO 8601 week numbers will be affected by this problem.

Peter van Ooijen and Shawn Steele has different solutions to this problem.

Here's a compact solution

private static int WeekOfYearISO8601(DateTime date)
{
    var day = (int)CultureInfo.CurrentCulture.Calendar.GetDayOfWeek(date);
    return CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(date.AddDays(4 - (day == 0 ? 7 : day)), CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
}

It's been tested for the following dates

var datesAndISO8601Weeks = new Dictionary<DateTime, int>
                        {
                            {new DateTime(2000, 12, 31), 52},
                            {new DateTime(2001, 1, 1), 1},
                            {new DateTime(2005, 1, 1), 53},
                            {new DateTime(2007, 12, 31), 1},
                            {new DateTime(2008, 12, 29), 1},
                            {new DateTime(2010, 1, 3), 53},
                            {new DateTime(2011, 12, 31), 52},
                            {new DateTime(2012, 1, 1), 52},
                            {new DateTime(2013, 1, 2), 1},
                            {new DateTime(2013, 12, 31), 1},
                        };

foreach (var dateWeek in datesAndISO8601Weeks)
{
    Debug.Assert(WeekOfYearISO8601(dateWeek.Key) == dateWeek.Value, dateWeek.Key.ToShortDateString() + " should be week number " + dateWeek.Value + " but was " + WeekOfYearISO8601(dateWeek.Key));
}

same problem with the rest of scandinavia I think, I'm danish and I was wondering why my tests showed 31/12 to be week #53 too as my Google Calender states its week #1. — BerggreenDK, Jan 16 '13 at 01:01
Because it's easier to read and less repetitive than `Dictionary datesAndISO8601Weeks = new Dictionary();` The reason I do it for simple types is that it keeps the code consistent. — Jonas Elfström, Mar 10 '15 at 12:49
Can also add that using var forces you to choose good variable names. — user1339253, Jan 22 '20 at 15:22

score 5 · Answer 2 · answered Sep 30 '09 at 11:38

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

score 4 · Answer 3 · answered Sep 30 '09 at 11:40

Check out GetWeekOfYear on MSDN has this example:

using System;
using System.Globalization;


public class SamplesCalendar  {

   public static void Main()  {

      // Gets the Calendar instance associated with a CultureInfo.
      CultureInfo myCI = new CultureInfo("en-US");
      Calendar myCal = myCI.Calendar;

      // Gets the DTFI properties required by GetWeekOfYear.
      CalendarWeekRule myCWR = myCI.DateTimeFormat.CalendarWeekRule;
      DayOfWeek myFirstDOW = myCI.DateTimeFormat.FirstDayOfWeek;

      // Displays the number of the current week relative to the beginning of the year.
      Console.WriteLine( "The CalendarWeekRule used for the en-US culture is {0}.", myCWR );
      Console.WriteLine( "The FirstDayOfWeek used for the en-US culture is {0}.", myFirstDOW );
      Console.WriteLine( "Therefore, the current week is Week {0} of the current year.", myCal.GetWeekOfYear( DateTime.Now, myCWR, myFirstDOW ));

      // Displays the total number of weeks in the current year.
      DateTime LastDay = new System.DateTime( DateTime.Now.Year, 12, 31 );
      Console.WriteLine( "There are {0} weeks in the current year ({1}).", myCal.GetWeekOfYear( LastDay, myCWR, myFirstDOW ), LastDay.Year );

   }

}

score 1 · Answer 4 · answered Sep 30 '09 at 11:37

1

My.Computer.Info.InstalledUICulture.DateTimeFormat.Calendar.GetWeekOfYear(yourDateHere, CalendarWeekRule.FirstDay, My.Computer.Info.InstalledUICulture.DateTimeFormat.FirstDayOfWeek)

Something like this...

answered Sep 30 '09 at 11:37

Bobby

11,419
5
44
69

score 1 · Answer 5 · edited Sep 05 '14 at 12:59

1

I know this is late, but since it came up in my search I thought I would throw a different solution in. This was a c# solution.

(int)(Math.Ceiling((decimal)startDate.Day / 7)) + (((new DateTime(startDate.Year, startDate.Month, 1).DayOfWeek) > startDate.DayOfWeek) ? 1 : 0);

edited Sep 05 '14 at 12:59

Kristian Nissen

1,153
2
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29

answered Mar 20 '13 at 16:31

GAdams

11
1

This returns "5" for the date 29/12/2016 - it should return 52 – DNKROZ Jan 16 '17 at 13:49

score 1 · Answer 6 · answered Sep 20 '15 at 11:13

If you want the ISO 8601 Week Number, in which weeks start with a monday, all weeks are seven days, and week 1 is the week containing the first thursday of the year, this may be a solution.

Since there doesn't seem to be a .Net-culture that yields the correct ISO-8601 week number, I'd rater bypass the built in week determination alltogether, and do the calculation manually, instead of atempting to correct a partially correct result.

What I ended up with is the following extension method:

public static int GetIso8601WeekNumber(this DateTime date)
{    var thursday = date.AddDays(3 - ((int)date.DayOfWeek + 6) % 7);
     return 1 + (thursday.DayOfYear - 1) / 7;
}

First of all, ((int)date.DayOfWeek + 6) % 7) determines the weekday number, 0=monday, 6=sunday.

date.AddDays(-((int)date.DayOfWeek + 6) % 7) determines the date of the monday preceiding the requested week number.

Three days later is the target thursday, which determines what year the week is in.

If you divide the (zero based) day-number within the year by seven (round down), you get the (zero based) week number in the year.

In c#, integer calculation results are round down implicitly.

Arif Dewi · Answer 7 · 2014-06-11T10:09:03.077

var cultureInfo = CultureInfo.CurrentCulture; var calendar = cultureInfo.Calendar;

        var calendarWeekRule = cultureInfo.DateTimeFormat.CalendarWeekRule;
        var firstDayOfWeek = cultureInfo.DateTimeFormat.FirstDayOfWeek;
        var lastDayOfWeek = cultureInfo.LCID == 1033 //En-us
            ? DayOfWeek.Saturday
            : DayOfWeek.Sunday;

        var lastDayOfYear = new DateTime(date.Year, 12, 31);

         //Check if this is the last week in the year and it doesn`t occupy the whole week
        var weekNumber = calendar.GetWeekOfYear(date, calendarWeekRule, firstDayOfWeek);
        return weekNumber == 53 && lastDayOfYear.DayOfWeek != lastDayOfWeek 
               ? 1 
               : weekNumber;

It works well both for US and Russian cultures. ISO 8601 also will be correct, `cause Russian week starts at Monday.

How can I calculate/find the week-number of a given date?

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