The default minimum date for the RadDatePicker is 01/01/1980. So, when you try to assign the value this than this date you will get the below error: if you will set the Mindate of RadDatePicker to a minimum range than this will fix the issue ID ="RadDateofBirth" DateInput-DateFormat ="dd/MM/yyyy" MinDate =" 01/01/1940 " DateInput-DisplayDateFormat ="dd/MM/yyyy" runat ="server"> This will solved the issue.