Calendar.GetWeekOfYear Method

  using System;
using System.Globalization;

public class Example
{
   public static void Main()
   {
            DateTimeFormatInfo dfi = new DateTimeFormatInfo();
            DateTime date1 = new DateTime(2011, 6, 10);
            System.Globalization.Calendar cal = dfi.Calendar;

            Console.WriteLine("{0}", cal.GetWeekOfYear(date1, CalendarWeekRule.FirstFullWeek, dfi.FirstDayOfWeek).ToString());       
   }
}

//using example above can show the week number + 1. The above shows the week number! 
//shows week 26

Calendar.GetWeekOfYear(new DateTime(2008,12,31), dfi.CalendarWeekRule, dfi.FirstDayOfWeek) returns 53 . This should be week 1 of 2009...

For a workaround I found this online : http://www.tech-archive.net/Archive/DotNet/microsoft.public.dotnet.languages.csharp/2008-11/msg01796.html

DateTimeFormatInfo.CurrentInfo.FirstDayOfWeek = Sunday
DateTimeFormatInfo.CurrentInfo.CalendarWeekRule = FirstDay

CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(dateTime,dfi.CalendarWeekRule, dfi.FirstDayOfWeek) returns

for {1/2/2010 12:00:00 AM} - 1st week
for {12/27/2009 12:00:00 AM} - 53rd week.

That's weird.

Windows 2003, Outlook 2007, .NET 3.5 SP1.

Not a Bug: This Is the Expected Behavior

CalendarWeekRule.FirstDay means that January 1 will always be the first day of the week, and that there will be 53 weeks in a year. The number of days in the first week will be the number of days from the January 1 to the first day of the week as defined by the calendar week rule. If the year begins on the first day of the week as defined by the calendar week rule, the first week will have 7 days. For non-leap years, if the first week has 7 days, the last week will have 1 day; otherwise, the number of days in the last week will equal 1 + (7 - daysInFirstWeek). In this case, January 1, 2010, was a Friday, so the first week of 2010 was January 1-January 2. In 2009, January 1 was a Thursday, so the first week of 2009 was January 1-January 3. Because the first week had 3 days, the last week (week 53) had 5 days, from 12/27/2009 to 12/31/2009.

You can determine the date ranges of each week of the year using code like the following. The parameters can be modified to see how other calendar week rules and first days of the week affect the date ranges of weeks and the number of weeks in the year. Here is the Visual Basic code:

Dim startDate As Date = #1/1/2009#
Dim cal As Calendar = CultureInfo.CurrentCulture.Calendar
Dim thisWeek, currentWeek As Integer

Dim thisDay As DateTime = startDate
Do
   thisWeek = cal.GetWeekOfYear(thisDay, CalendarWeekRule.FirstDay, DayOfWeek.Sunday)
   If thisWeek > currentWeek Or thisWeek < currentWeek Then
      If currentWeek > 0 Then Console.WriteLine("{0:d}", thisDay.AddDays(-1))
      currentWeek = thisweek
      Console.Write("{0}: {1:d} - ", currentWeek, thisDay)
   End If 
   thisDay = thisDay.AddDays(1) 
Loop While thisDay <= #1/10/2010# 

And here is the C# code:

DateTime startDate = new DateTime(2009, 1, 1);
DateTime endDate = new DateTime(2010, 1, 10);
Calendar cal = CultureInfo.CurrentCulture.Calendar;
int thisWeek = 0;
int currentWeek = 0;

DateTime thisDay = startDate;
do {
   thisWeek = cal.GetWeekOfYear(thisDay, CalendarWeekRule.FirstDay, DayOfWeek.Sunday);
   if (thisWeek > currentWeek || thisWeek < currentWeek) {
      if (currentWeek > 0) Console.WriteLine("{0:d}", thisDay.AddDays(-1));
      currentWeek = thisWeek;
      Console.Write("{0}: {1:d} - ", currentWeek, thisDay);
   } 
   thisDay = thisDay.AddDays(1); 
} while (thisDay <= endDate); 

--Ron Petrusha
CLR Developer Content Team
Microsoft Corporation