JAVASCRIPT validator is not working properly

Question

I used a JavaScript validator to compare month and year.

$.validator.addMethod("lessThan", function(value, element, params) {
            var end_year_id   = "1-endDateYear";
            var start_year_value = "";
            if(element.id === end_year_id) 
                start_year_value = $("#1-startDateYear").val();
            else 
                start_year_value = $("#2-startDateYear").val();

            if (!(isNaN(start_year_value) || isNaN(value))) {
               return new Date(value) > new Date(start_year_value);
            }
            return isNaN(value) && isNaN(start_year_value) || (Number(value) > Number(start_year_value)); 
        });

It displays an error message when I choose an other month in the same year :( ie. I choose january-2013 and november-2013 its displaying error that end year must be greater than start year. I want a user can select different month in same year. This js is not working.