Is there anything that stops the DateTime AddDays() method that doesn't run within a while loop. I have this simple bit of code; DateTime last_day = monthCalendar2.SelectionRange.End; DateTime first_day = new DateTime(year, month, day); //Insert dates into vector while (first_day != last_day) { dates.Add(first_day); first_day.AddDays(1); } I step through the program and first_day never changes, anyone know why?!