Answer by kam for Detecting an "invalid date" Date instance in JavaScript

Date object to string is more simple and reliable way to detect if both fields are valid date. e.g. If you enter this "-------" to the date input field. Some of the above answers won't work.

jQuery.validator.addMethod("greaterThan", 

    function(value, element, params) {
        var startDate = new Date($(params).val());
        var endDate = new Date(value);

        if(startDate.toString() === 'Invalid Date' || endDate.toString() === 'Invalid Date') {
            return false;
        } else {
            return endDate > startDate;
        }
    },'Must be greater than {0}.');