compare dates

i have 2 textboxes for dates, a beginning date and ending date, am using ajax extender for popup calendar for both, and a validator to check for a valid date entered, and using compare to test if the end date is greater than beginning date. it works great if there are 2 dates. however if there is only 1 of the 2 dates the compare does not work. is this too complex for client side validators? do I need to evaluate in code behind, to determine if there is one date only? 
<asp:TextBox CssClass="textNotReq" id="_begdate01" maxlength="10" columns="10" runat="server" Text="" /> <asp:CalendarExtender ID="_begdate01CalExt" runat="server" TargetControlID="_begdate01" Format="MM/dd/yyyy" ></asp:CalendarExtender> <asp:CompareValidator id="_begdate01DateValid" runat="server" Type="Date" Operator="DataTypeCheck" ...