Testing and message for attempt to create new event before current time and date

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>

I am double clicking a free area. As such, there is no start or end

The double click event is handled by TimeRangeDoubleClick but otherwise it's the same. The start and end variables will hold the cell start and end, respectively.