Best option would be
Add a compare validator to the web form. Set its controlToValidate. Set its Type property to Date. Set its operator property to DataTypeCheck eg:
<asp:CompareValidator
id="dateValidator" runat="server"
Type="Date"
Operator="DataTypeCheck"
ControlToValidate="txtDatecompleted"
ErrorMessage="Please enter a valid date.">
</asp:CompareValidator>