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
PageMethods.CheckDateTime("yourargument");
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.