CODEWITHSUNDEEP
c#asp.netdate
cllpse
cllpse

Reputation: 21727

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

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

Upvotes: 34

Views: 66708

Answers (8)

Ramon Smits
Ramon Smits

Reputation: 2618

Since .net Core 3.0 till now (.net 9) a new type exists ISOWeek.

Most comments on this post are for legacy .net. Don't use that for new projects!

Provides static members to support the ISO week date that is part of the ISO 8601 date and time standard issued by the International Organization for Standardization (ISO).

https://learn.microsoft.com/en-us/dotnet/api/system.globalization.isoweek

DateTime date = new DateTime(2024, 12, 30);
int weekNumber = ISOWeek.GetWeekOfYear(date);

Upvotes: 0

Jonas Elfström
Jonas Elfström

Reputation: 31428

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));
}

Upvotes: 77

realbart
realbart

Reputation: 3965

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.

Upvotes: 1

GAdams
GAdams

Reputation: 11

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);  

Upvotes: 1

Arif Dewi
Arif Dewi

Reputation: 2272

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.

Upvotes: 0

SwDevMan81
SwDevMan81

Reputation: 49978

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 );

   }

}

Upvotes: 4

Andriy Tkach
Andriy Tkach

Reputation: 1471

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

Upvotes: 5

Bobby
Bobby

Reputation: 11576

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

Something like this...

Upvotes: 1

Related Questions