Store all DB datetime fields in UTC

[lousyfool]

Currently many (= most?) datetime fields are stored in the DB in the user's timezone as set in their J! account or, if set to "default" there or if it's a guest in the frontend, in J!'s global system timezone.
That's a big no-no, typically messing up both backend and frontend display, add/edit etc etc, basically J!'s native time system.

In the database, all datetime fields must be stored in UTC time. Always and everywhere, no matter set system or user timezones.

Also see https://www.joomlaworks.net/forum/k2-en/63669-the-timezone-conundrum