public static int ISOWeekNumber(DateTime dt)
{
int yyyy = dt.Year;
int mm = dt.Month;
int dd = dt.Day;
// Declare other required variables
int jan1WeekDay;
int weekNumber = 0;
int weekDay = 0;
int i, j, k, l, m, n;
int[] mnth = new int[12] { 0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334 };
int yearNumber;
// Set DayofYear Number for yyyy mm dd
int dayOfYearNumber = dd + mnth[mm - 1];
// Increase of Dayof Year Number by 1, if year is leapyear and month is february
if ((IsLeapYear(yyyy)) && (mm == 2)) dayOfYearNumber += 1;
// Find the Jan1WeekDay for year
i = (yyyy - 1) % 100;
j = (yyyy - 1) - i;
k = i + i / 4;
jan1WeekDay = 1 + (((((j / 100) % 4) * 5) + k) % 7);
// Calcuate the WeekDay for the given date
l = dayOfYearNumber + (jan1WeekDay - 1);
weekDay = 1 + ((l - 1) % 7);
// Find if the date falls in YearNumber set WeekNumber to 52 or 53
if ((dayOfYearNumber <= (8 - jan1WeekDay)) && (jan1WeekDay > 4))
{
yearNumber = yyyy - 1;
if ((jan1WeekDay == 5) || ((jan1WeekDay == 6) && (jan1WeekDay > 4))) weekNumber = 53;else weekNumber = 52;
}
else yearNumber = yyyy;
// Set WeekNumber to 1 to 53 if date falls in YearNumber
if (yearNumber == yyyy)
{
if (IsLeapYear(yyyy) == true) m = 366;
else m = 365;
if ((m - dayOfYearNumber) < (4 - weekDay))
{
yearNumber = yyyy + 1;
weekNumber = 1;
}
}
if (yearNumber == yyyy)
{
n = dayOfYearNumber + (7 - weekDay) + (jan1WeekDay - 1);
weekNumber = n / 7;
if (jan1WeekDay > 4) weekNumber -= 1;
}
return (weekNumber);
}
public static bool IsLeapYear(int yyyy)
{
return ((yyyy % 4 == 0 && yyyy % 100 != 0) || (yyyy % 400 == 0));
}