Adding a new event with client-side immediate update

After you create the new event in newcalevent.php, you need to send the event details (including the id) back to the main page using DayPilot.Modal.close():

var eventdata = {
  id: ...,
  start: ...
  // ...
};
DayPilot.Modal.close({event: eventdata, ...});

This object will be available in onClosed as args.result.event so you can use it to create the new event:

modal.onClosed = function(args) {
  dp.clearSelection();
  var data = args.result;
  if (data && data.result === "OK") { 
    dp.events.add(new DayPilot.Event(data.event));
    dp.message(data.message); 
  }
};

I was using the following from another sample in my newcalevent to get that information

        $("#f").submit(function (ev) {
            // make sure it's not submitted using the default mechanism
            ev.preventDefault();
            
            // normalize the date values
            $("#start").val(startPicker.date.toString("yyyy-MM-dd"));
            $("#end").val(endPicker.date.toString("yyyy-MM-dd"));
            
            // submit using AJAX
            var f = $("#f");
		    $.post("backend_create.php", f.serialize(), function (result) {
                close(eval(result));
            });
            
        });

The code you provided is being used as such:

dp.onTimeRangeSelected = function (args) {
	var modal = new DayPilot.Modal();
	modal.onClosed = function(args) {
		dp.clearSelection();
//		alert('returned data id:' + args.result.id + ' Start=' + args.result.start );
		var data = args.result;
		if (data && data.result === "OK") { 
			dp.events.add(new DayPilot.Event(data.event));
			dp.message(data.message); 
		}
	};
	modal.showUrl("newcalevent.php?start=" + args. start + "&end=" + args.end + "&resource=" + args.resource + "&userid=" + '<?php echo $UserID;?>');
};

HOWEVER, despite seeing the data id being returned and the message being shown on the screen after the add, the new item does not show on the calendar.

You should modify "backend_create.php" to return the full event data object (including start, end, etc.) as "event" property in the return JSON. Because that's what is passed back to onClosed:

$.post("backend_create.php", f.serialize(), function (result) {  
  // result is the backend_create.php return value
  close(eval(result));
});

Exactly my problem was that I incorrectly assumed the submitted data was being passed all the way through the process from the modal back to the originating page.