bottega/simple-membership/views/admin_member_form_common_js.php

<script>
    jQuery(document).ready(function ($) {
        $('#subscription_starts').datepicker({dateFormat: 'yy-mm-dd', changeMonth: true, changeYear: true, yearRange: "-100:+100"});
        $('#member_since').datepicker({dateFormat: 'yy-mm-dd', changeMonth: true, changeYear: true, yearRange: "-100:+100"});
    });
</script>