Validate that end date is greater than start date with jQuery

Question

How do I check/validate in jQuery whether end date [textbox] is greater than start date [textbox]?

Answer 1

So I needed this rule to be optional and none of the above suggestions are optional. If I call the method it is showing as required even if I set it to needing a value.

This is my call:

  $("#my_form").validate({
    rules: {
      "end_date": {
        required: function(element) {return ($("#end_date").val()!="");},
        greaterStart: "#start_date"
      }
    }
  });//validate()

My addMethod is not as robust as Mike E.'s top rated answer, but I'm using the JqueryUI datepicker to force a date selection. If someone can tell me how to make sure the dates are numbers and have the method be optional that would be great, but for now this method works for me:

jQuery.validator.addMethod("greaterStart", function (value, element, params) {
    return this.optional(element) || new Date(value) >= new Date($(params).val());
},'Must be greater than start date.');