Solved:
<asp:comparevalidator id="compareValidatorDate" runat="server" controltovalidate="datePickerED$txtDatePicker" controltocompare="datePickerSD$txtDatePicker" cultureinvariantvalues="true" xmlns:asp="#unknown">
ErrorMessage="End date should be greater than start date" Operator="GreaterThanEqual" SetFocusOnError="false" Type="String" Display="Dynamic" EnableClientScript="true" Font-Size="Small" ForeColor="Red"></asp:comparevalidator>