Thank you for writing.
By default, the EditAppointmentDialog
loads the data coming from the associated appointment. According to the provided information, I suppose that you need to change the date shown in the edit dialog. For this purpose, you can create a custom EditAppointmentDialog
and override its LoadSettingsFromEvent
method. This method is responsible for loading the appointment's data in the dialog. You can manage the controls according to your needs. The following help article demonstrates a sample approach how to create a custom edit dialog and show it in RadScheduler: https://docs.telerik.com/devtools/winforms/scheduler/appointments-and-dialogs/adding-a-custom-field-to-the-editappointment-dialog
I hope this information helps. Should you have further questions I would be glad to help.