Workaround:

            this.radDateTimePicker1.DateTimePickerElement.ShowTimePicker = true; 
            RadDateTimePickerCalendar calendarBehavior = this.radDateTimePicker1.DateTimePickerElement.GetCurrentBehavior() as RadDateTimePickerCalendar;
            calendarBehavior.PopupControl.Opened += PopupControl_Opened;
            calendarBehavior.PopupControl.Opening+=PopupControl_Opening;
            calendarBehavior.PopupControl.Closing+=PopupControl_Closing;
            calendarBehavior.PopupControl.Closed+=PopupControl_Closed;