function resultOfValidation(result);
{
    return result;
}

function IsValidDate()
{
    PageMethods.ComplicatedValidation(resultOfValidation);
}

[WebMethod]
public static bool ComplicatedValidation()
{
    return true;
}

function IsDateTimeAvailable()
{
    var result= PageMethods.ComplicatedValidation();
}

function GetValue()
{
    return PageMethods.GetValueFromServer(
     function(result)
       {
          // The result that is returned from server
          //Now do what ever you would like to do.
       }
    );
}