crashing the program on a specific date C# : i use this code to get date

    String.Format("{0:yyyy/MM/dd}", Convert.ToDateTime(calender.Text))

it work correct .
but cant get 3 Special data so my program crashing.
in the day "31" and Month "2" , "4" , "6" for example `"1397/06/31"`or `"1397/04/31"`
 
how to fix it?
