DateTime dtLast = new DateTime(Convert.ToInt32(LastMenses.Year), Convert.ToInt32(LastMenses.Month), Convert.ToInt32(LastMenses.Day));

DateTime dtThis = new DateTime(Convert.ToInt32(Year), Convert.ToInt32(thisMonth), Convert.ToInt32(Day));

int interval = new TimeSpan(dtThis.Ticks - dtLast.Ticks).Days;