What I understand from your question is - Before posting the form, you'd like to check if the DOB field has a date that must not be greater than today's date.. If this is correct, then try this, Hope this help -

<script type=text/javascript>

    function checkDOB() {

        var dateString = document.getElementById('id_dateOfBirth').value;

        var myDate = new Date(dateString);

        var today = new Date();

        if ( myDate > today ) { 

            $('#id_dateOfBirth').after('<p>You cannot enter a date in the future!.</p>');

            return false;

        }

        return true;

    }

</script>