How to validate start dates and end dates

I have a form related to contracts. There are two fields for the contract terms: start date and end date.

My wish for the validation of these two fields:

1) That users are alerted to enter a start date before they enter an end date

2) That users are alerted to correct the end date if it comes before the start date

Here was my attempt:

event.rc = true;

if (event.value) {

    var startDateString = this.getField("StartDate_af_date").value;

    if (startDateString=="") {

        app.alert("You must first fill-in the contract start date.");

        event.rc = false;

    } else {

        var startDate = this.getField("StartDate_af_date").value;

        var endDate = event.value;

        if (endDate>startDate) {

            app.alert("End date of the contract must be AFTER start date. Please correct.");

            event.rc = false;

        }   

    }

}

No matter what end date is entered, the alert pops up.

I'm a self-taught scripter and cobbled this together from another forum answer. I know I am missing something critical here but my research hasn't helped me.

Thank you!