Repro steps:

1. Add a JIRA Issue dates event.
2. Select a field in the Single date list
3. Edit one of the JIRA issue dates event.
4. In the date picker, enter a date after the original date. Do not use the date picker calendar, just type the date.
5. Click OK to save the changes.

You receive an error saying: The start date must not be after the end date; but you are not able to edit the end date, as it is a single date event and the end date field is grayed out.