$('#datepicker').datepicker().change(evt => {
var selectedDate = $('#datepicker').datepicker('getDate');
var now = new Date();
now.setHours(0,0,0,0);
if (selectedDate < now) {
console.log("Selected date is in the past");
} else {
console.log("Selected date is NOT in the past");
}
});
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<script src="https://code.jquery.com/ui/1.12.1/jquery-ui.min.js"></script>
<input type="text" id="datepicker" name="event_date" class="datepicker">