First you need to take reference of ScriptManager control from your page.
<asp:scriptmanager id="scriptmanager" runat="server"></asp:scriptmanager>
If it is already there to your masterpage then no need. Then from your mobilevalidate method you can call server side method CheckDateTime like as follows
You can also called that serverside method using jquery or other client side library like prototype. If you search online then you will find many codesample.