problem with the leap year formula

hi, this is just a part of my function its not giving me actual result its accepting the year which are not leap year i.e 29 feb for 1983 is not a valid date
if (dropDOBMonth.SelectedItem.Text.ToLower().Contains("feb"))
{
if (int.Parse(dropDOBYear.SelectedItem.Value) % 400 == 0 || (int.Parse(dropDOBYear.SelectedItem.Value) % 100 != 0 && int.Parse(dropDOBYear.SelectedItem.Value) % 4 == 0))
DayLength = 29;

else

DayLength = 28;

}
...