My LogiX is Undeniable.
DateTime.DaysInMonth(int year, int month);
//or
static int GetDaysInMonth(int year, int month)
{
DateTime dt1 = new DateTime(year, month, 1);
DateTime dt2 = dt1.AddMonths(1);
TimeSpan ts = dt2 - dt1;
return (int)ts.TotalDays;
}