Getting the week number of the year from the DateTime object can be implemented proprietary-ly but Microsoft does provide a way to get this calculated with a built-in method.
Here is the way you can achieve this:
static void Main(string[] args) { DateTime inputDate = DateTime.Now; CultureInfo currentCulture = CultureInfo.CurrentCulture; var weekNum = currentCulture.Calendar.GetWeekOfYear( inputDate, CalendarWeekRule.FirstDay, DayOfWeek.Monday); Console.WriteLine("Year: {0} Week: {1}", inputDate.Year, weekNum); }