The current workaround is to make sure that both the day and month include leading zeros in the ShortDatePattern:

            this.DatePicker.Culture.DateTimeFormat.ShortDatePattern = "yyyy.MM.dd";

For example if you have 12:00 selected and input 15:03 in the WatermarkTextbox- 12:00 item of the Clock is still selected

When FlowDirection is RTL, navigating in Calendar and Clock is inverted- pressing left arrow moves the selection to the right and vice versa.

When StartTime = EndTime all hours are visable (12 am - 11pm)
Workaround:
Select EndTime in one hour interval from StartTime (10:00 - >10:01). In this situation only 10:00 will be visible

RadDateTimePicker ControlTemplate: PopupPlacementTarget in RadDropDownButton is not respected when FlowDirection of RadDateTimePicker is RTL. PopupWrapper issue.

RadDateTimePicker cannot parse datetime string when Windows date format is set to "Chinese (Traditional, Taiwan)".