Issue Summary

When creating a recurring event with an "end repeat date", the event created in the database not formatted properly. This leads to errors like this in the atlassian-confluence.log:

2021-08-17 11:08:15,856 ERROR [http-nio-6749-exec-9] [extra.calendar3.model.SubCalendarEvent] toJson Unable to create a JSON object based on this object
 -- referer: http://localhost:6749/c749/calendar/mycalendar.action | url: /c749/rest/calendar-services/1.0/calendar/events.json | traceId: de95e552e811c387 | userName: admin
java.lang.IllegalArgumentException: Invalid format: "20211231" is too short

Based on the below test, Team Calendars versions that are above the 7.0.14 (bundled version) are affected by this

Steps to Reproduce

1. Install Confluence 7.4.9 and apply a DC license
2. Install Team Calendars 7.0.17 manually.
3. Create a calendar
4. Create a recurring event like this:
   
5. Save the event

When you check the RECURRENCE_RULE field for this event in the database:

Expected Results

RECURRENCE_RULE|FREQ=WEEKLY;UNTIL=20211231T230000Z;INTERVAL=1;BYDAY=TH

Actual Results

RECURRENCE_RULE|FREQ=WEEKLY;UNTIL=20211231;INTERVAL=1;BYDAY=TH

Workaround

The record should be manually fixed in the database. RECURRENCE_RULE format should be changed from UNTIL=YYYYMMDD to UNTIL=YYYYMMDDTHHMMSSZ.

Please ensure that Confluence is not running during the database modification to avoid potential data inconsistency.