public static int WeekNumber(DateTime fromDate)
{
// Get jan 1st of the year
DateTime startOfYear = fromDate.AddDays(-fromDate.Day + 1).AddMonths(-fromDate.Month + 1);
// Get dec 31st of the year
DateTime endOfYear = startOfYear.AddYears(1).AddDays(-1);
// ISO 8601 weeks start with Monday
// The first week of a year includes the first Thursday, i.e. at least 4 days
// DayOfWeek returns 0 for Sunday up to 6 for Saturday
int[] iso8601Correction = { 6, 7, 8, 9, 10, 4, 5 };
int nds = fromDate.Subtract(startOfYear).Days + iso8601Correction[(int)startOfYear.DayOfWeek];
int wk = nds / 7;
switch (wk)
{
case 0:
// Return weeknumber of dec 31st of the previous year
return WeekNumber(startOfYear.AddDays(-1));
case 53:
// If dec 31st falls before thursday it is week 01 of next year
if (endOfYear.DayOfWeek < DayOfWeek.Thursday)
return 1;
else
return wk;
default: return wk;
}
}