一年有52周零1天，如果是一圈年(52 x 7 = 364)则为2天。2012-12-31是第53周，一周只有2天，因为2012年是一圈年。

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);

var weekNumber = calendar.GetWeekOfYear(date, calendarWeekRule, firstDayOfWeek);

 //Check if this is the last week in the year and it doesn`t occupy the whole week
return weekNumber == 53 && lastDayOfYear.DayOfWeek != lastDayOfWeek 
       ? 1  
       : weekNumber;

它对美国文化和俄罗斯文化都适用。ISO 8601也是正确的，因为俄罗斯周从周一开始。

  DateTimeFormatInfo dfi = DateTimeFormatInfo.CurrentInfo;
  DateTime date1 = new DateTime(2011, 1, 1);
  Calendar cal = dfi.Calendar;

  Console.WriteLine("{0:d}: Week {1} ({2})", date1, 
                    cal.GetWeekOfYear(date1, dfi.CalendarWeekRule, 
                                      dfi.FirstDayOfWeek),
                    cal.ToString().Substring(cal.ToString().LastIndexOf(".") + 1));      

从上面il_guru的代码到Powershell端口:

function GetWeekOfYear([datetime] $inputDate)
{
   $day = [System.Globalization.CultureInfo]::InvariantCulture.Calendar.GetDayOfWeek($inputDate)
   if (($day -ge [System.DayOfWeek]::Monday) -and ($day -le [System.DayOfWeek]::Wednesday))
   {
      $inputDate = $inputDate.AddDays(3)
   }

   # Return the week of our adjusted day
   $weekofYear = [System.Globalization.CultureInfo]::InvariantCulture.Calendar.GetWeekOfYear($inputDate, [System.Globalization.CalendarWeekRule]::FirstFourDayWeek, [System.DayOfWeek]::Monday)
   return $weekofYear
}

正如在这个MSDN页面中所指出的，ISO8601周和。net周编号之间有轻微的差异。

你可以参考MSDN博客中的这篇文章来获得更好的解释:“微软。net中的ISO 8601年度周格式”

简单地说，. net允许将周划分为年，而ISO标准不允许。 在本文中，还提供了一个简单的函数，用于获取每年最后一周的正确ISO 8601周数。

更新下面的方法实际上为2012-12-31返回1，这在ISO 8601中是正确的(例如德国)。

// This presumes that weeks start with Monday.
// Week 1 is the 1st week of the year with a Thursday in it.
public static int GetIso8601WeekOfYear(DateTime time)
{
    // Seriously cheat.  If its Monday, Tuesday or Wednesday, then it'll 
    // be the same week# as whatever Thursday, Friday or Saturday are,
    // and we always get those right
    DayOfWeek day = CultureInfo.InvariantCulture.Calendar.GetDayOfWeek(time);
    if (day >= DayOfWeek.Monday && day <= DayOfWeek.Wednesday)
    {
        time = time.AddDays(3);
    }

    // Return the week of our adjusted day
    return CultureInfo.InvariantCulture.Calendar.GetWeekOfYear(time, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
} 

在。net 3.0及以后版本中，你可以使用isoweek . getweekofdate方法。

请注意，年+周数格式中的年份可能与DateTime中的年份不同，因为有些星期跨越了年份边界。