Try this, here we use the Ajax calander control to get the input in dd/mm/yyyy format and then use the compare validator
<asp:TextBox ID="txtStart" runat="server"></asp:TextBox>
<cc1:CalendarExtender ID="txtStart_CalendarExtender" runat="server"
Enabled="True" TargetControlID="txtStart">
</cc1:CalendarExtender>
<asp:CompareValidator ID="CompareValidator1" runat="server"
ControlToCompare="txtEnd" ControlToValidate="txtStart"
ErrorMessage="CompareValidator"></asp:CompareValidator>
</div>
<p>
<asp:TextBox ID="txtEnd" runat="server"></asp:TextBox>
<cc1:CalendarExtender ID="txtEnd_CalendarExtender" runat="server"
Enabled="True" TargetControlID="txtEnd">
</cc1:CalendarExtender>
</p>
<asp:Button ID="Button1" runat="server" Text="Button" />
<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>