jQuery validator custom "greater than" rule

Question

I'm trying to use a custom validation rule with jquery.validator. In the form there are 2 text fields 'startdate' and 'enddate', containing date strings (using jquery.ui datepicker). The date in the 'enddate' field has to be "bigger than" the one in startdate.

I found the following 'greaterThan' custom function in the additional file that comes with validator:

 $.validator.addMethod( "greaterThan", function( value, element, param ) {
            var target = $( param );

            if ( this.settings.onfocusout && target.not( ".validate-greaterThan-blur" ).length ) {
                target.addClass( "validate-greaterThan-blur" ).on( "blur.validate-greaterThan", function() {
                    $( element ).valid();
                } );
            }

            return value > target.val();
        }, "Please enter a greater value." );

Now I'm not sure how to do write the 'rules' part for this field. Thanks for help!! Regards

I tried something like this:

rules {
  enddate:{
      greaterThan: true,
      value: startdate
  }
}

but that doesn't work...