Adding to what VJ Reddy explained, I usually prefer to use
TimeSpan[
^]. It gives you flexibility when you want to have more accurate calculations. For eg. 30 days, 10 hours and 45 mins :)
DateTime date_Of_End = new DateTime(2012, 6, 02);
int duration = 30;
TimeSpan tsDay = newTimeSpan(duration, 0, 0, 0);
DateTime date_Of_Bigin = date_Of_End.Subtract(tsDay);