How to automatically change countdown

i got countdown script for one months so how i can do when countdown till zero then it should be automatically change to new countdown date for next months? without for me to manually change it every months myself.

its javascript

<script type="text/javascript">
    jQuery(document).ready( function($){
        var newDate = new Date(2016, 4, 21);
        newDate.setHours(newDate.getHours() + 18);
        $('#countdown-ex1').countdown({until: newDate});
    });
</script>

and Html

<div id="countdown-ex1" class="countdown countdown-large coming-soon divcenter bottommargin" style="max-width:700px;"></div>