Editing repeated Team Calendar events causes events next to the day to be affected

Confluence 6.12.0
Team Calendar 6.0.20

Issue seems to be related to the issues below.

1. CONFSERVER-55165
2. CONFSERVER-55287
3. CONFSERVER-55654

Summary

When a weekly repeated event is created in Team Calendar, editing a single event affects nearby events as well, in some cases moving the event to the previous day, or if an event is moved to the next day, the event that's on the next day gets shifted as well.

Steps to reproduce

1. Create a repeated event like the example below.
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2. The event should be look like something below.
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3. Now if we were to edit one of the event, by extending it like below.
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4. Refreshing the page will show that the event from the next day is moved to the edited day.
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5. If we were to instead move the event to the next day like below
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6. Refreshing the page will cause the event to move like this.
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7. There can be more variations of this issue if we were to move the events around more for example, once the events are as they are in Step 6 above, moving the event from the second day back up and refreshing the page will bring the event back down to how it was in the screenshot again, unless we were to move the event from the next day over, and move the event from the second day to the first day instead, and only then can it be returned to how it was in the beginning, like the screenshot in Step 2. 

Error in the logs

2018-11-26 21:57:05,218 WARN [http-nio-8090-exec-9] [extra.calendar3.util.TimeZoneUtil] tryParseDateTimeStringForEventEdit Unable to parse date null time 8:00 AM
 -- referer: http://localhost:8090/confluence/calendar/mycalendar.action | url: /confluence/rest/calendar-services/1.0/calendar/events.json | traceId: 87a321af2fbb072d | userName: root
2018-11-26 21:57:23,096 WARN [http-nio-8090-exec-5] [extra.calendar3.util.TimeZoneUtil] tryParseDateTimeStringForEventEdit Unable to parse date null time 8:00 AM

Expected behavior

1. Be able to edit individual events that were created as part of a reoccurring event, without affecting events from other days.

Actual behavior

1. Editing individual events that were created as part of a reoccurring event, affects neighboring events.