How to activate onValueChangeHandler for a DateBox using GAS

I have 2 dateboxes and added an onValueChangeHandler to both of them in order to update a flextable of events if a date will be changed. It works.

While loading (after doGet returns) I provide valid data for both dateboxes and would like the onValueChangeHandler to be executed. Unfortunally unlike checkboxes, dateboxes do not have a setValue(date, true); method .

   var onValueChangeShowEvents = app.createServerHandler('onValueChangeShowEvents');
   var dbxDateFrom  = app.createDateBox().setId(wid_dbxDateFrom).setWidth('85')
                              .addValueChangeHandler(onValueChangeShowEvents);
   var dbxDateUntil = app.createDateBox().setId(wid_dbxDateUntil).setWidth('85')
                              .addValueChangeHandler(onValueChangeShowEvents);
   var eventsTable  = app.createFlexTable().setId(wid_eventsTable);


function onValueChangeShowEvents(e)
{
var func = 'onValueChangeShowEvents';
Logger.log('in ' + func + '   source= ' + e.parameter.source + '   value= ' + e.parameter[e.parameter.source]);
   var app  = UiApp.getActiveApplication();

   var dbxDateFrom  = app.getElementById(wid_dbxDateFrom);
   var dbxDateUntil = app.getElementById(wid_dbxDateUntil);

   var dateFrom  = new Date(e.parameter[wid_dbxDateFrom]);
   var dateUntil = new Date(e.parameter[wid_dbxDateUntil]);
   dateUntil     = new Date(dateUntil.getTime() + (24 * 60 * 60 - 1) * 1000); // Full day until 23 u 59.59

   var cal       = CalendarApp.getCalendarsByName(MyCalender)[0]; // Retrieve the calender (verify !!)  
   var events    = cal.getEvents(dateFrom, dateUntil);              // All events within the timeframe
   var numEvents = events.length;

   var eventsTable = app.getElementById(wid_eventsTable);
   eventsTable.clear().setCellSpacing(1).setCellPadding(1);

//   eventsTable.setColumnStyleAttributes(0, { textAlign: "right" } ); // Does not work
   eventsTable.setStyleAttributes({ textAlign: "right" } );

   var numFields   = 5;
   var text        = '';
   var widthFields = [ '26', '85', '50', '50', '200'];

   for (var i=0; i<numEvents; i++)
   {
      var event       = events[i];
      var numRow      = i + 1;
//      var id          = event.getId();
      var dateStart   = event.getStartTime();
      var dateEnd     = event.getEndTime();
      var title       = event.getTitle();
//      var location    = event.getLocation();
//      var guests      = event.getGuestList();

      for (var j=0; j<numFields; j++)
      {  
         var fieldId  = evtPrefix + bell + numRow + bell + j; // Coding the fields
         var tbxField = app.createTextBox().setReadOnly(true).setStyleAttribute('fontWeight', 'bold')
                                .setId(fieldId).setName(fieldId).setWidth(widthFields[j]);
         if (j == 0)      text = numRow; // Must be changed into a button or anchor in order to allow show and update the event
         else if (j == 1) text = dateStart.format('yyyy-mm-dd'); // Uses http://blog.stevenlevithan.com/archives/date-time-format
         else if (j == 2) text = dateStart.format('HH:MM');
         else if (j == 3) text = dateEnd.format('HH:MM');
         else if (j == 4) text = title;

         tbxField.setText(text);
         eventsTable.setWidget(numRow, j, tbxField);
      }      
   };

   return app;
}

How can I initially activate my onValueChangeHandler from code?

Not sure why you couldn't assign values in the doGet directly but if you really want to be able to call your function from an external function (not triggered by a handler) you can call it with a valid e parameter. e is actually a JavaScript object with a few properties and is pretty easy to build.

var e = {};
var parameter = {};
parameter['wid_dbxDateFrom'] = 'someValue';
parameter['wid_dbxDateUntil'] = 'someOtherValue';
e['parameter'] = parameter;// I defined only 2 parameters but you can add others + source + whatever you want...
onValueChangeShowEvents(e);// call your function with the necessary parameters