Duedate field not consistent between 'dates' panel and 'activity' depending on TZ

Summary

A client has noticed that there are inconsistencies in the due date field from the "activity" panel on the issue view screen. More info in the notes.

Steps to Reproduce

1. Use an instance with a timezone of GMT+x (say, Amsterdam).
2. Log in as a user with the same timezone settings.
3. Change the "duedate" to any date - say, the 25th of December.
4. On the issue view screen, click on the "Activity" panel - you should see "<USER> changed the Due Date to '25/12/17' on <ISSUE>"
5. Change the user timezone - User > Profile > Preferences > click on the "edit" icon, change your timezone to somewhere west of your original timezone say, London - GMT+00) and save.
6. Go back to the issue, view "Activity", you will see "<USER> changed the Due Date to '24/12/17' on <ISSUE>".

Expected Results

The due date is expected to always be the same both on the "dates" and the "Activity" panels regardless of the user's timezone.

Actual Results

If the user is in a timezone west of the one configured on the server, the due date in "Activity" is one day less than that in the 'dates' panel

Notes

This problem happens because, on a DB level, we are storing a timestamp, not just a date - i.e. rather than storing "2017-12-25", we're also storing the hour, i.e. "2017-12-25 00:00:00". It goes without saying that midnight of any day in Amsterdam is 11 PM of the day before if you're in London, which is why the London user sees the day as the 24th whereas the issue info panel will show the day as the 25th.

Workaround

• You can set all customers' timezone "east" of the server's
• You can set the server's timezone to a place to the west of all users on the system
• You should be able to set the default user timezone east of the server's (haven't verified this one)