I have the datepicker set to MM-DD-YYYY in the TEC settings. When creating an event with a ONCE recurrence pattern, the datepicker works correctly until you SAVE the event. After saving the event and the screen refreshes, the series shows the datepicker entry as the default, YYYY-DD-MM, and then displays an error under the series.
It doesn’t appear to have a negative effect on the event, only a UI/UX issue.
See attached screenshots of before and after the save process.
Hope this helps.