I assume you want to get this on the basis of the Calender. For this you need System.Globalization
DateTime date1 = DateTime.Now;
DateTimeFormatInfo dinfo = DateTimeFormatInfo.CurrentInfo;
dinfo.Calendar.GetWeekOfYear(date1, CalendarWeekRule.FirstFullWeek, DayOfWeek.Monday)
Based on your need you have to set the Calender week rule and the first day of the week.
This gives you a week number for the calender. you can get the same for your other date, the difference is your weeks count
Hope this helps you.