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?!