It depends on how you are creating a new event. If it's directly on TimeRangeSelected it could look like this:

TimeRangeSelectedHandling="JavaScript"
TimeRangeSelectedJavaScript="check(start, end, resource)"

<script style="text/javascript">
function check(start, end, resource) {
  var now = new DayPilot.Date();
  // var today = now.getDatePart();
  
  if (end.getTime() < now.getTime()) {
    if (!(confirm('Are you sure that you want to create an event in the past?')) {
      return;
    }
  }
  dpc.timeRangeSelected(start, end, resource);
}
</script>