I have written this script StackExchange sites - convert dates to local timezone
This script applies to StackExchange sites stackoverflow.com, stackexchange.com, superuser.com, etc.

The timezone that the StackExchange sites use is UTC i.e. +0000 (source)
So, this script converts the dates to your local timezone, in both:

• tooltips 2015-12-14 14:11:13Z, and in
• date text like Dec 14 '15 at 14:11 or Dec 14 '15 at 14:11. superuser.com

It also recalculates them whenever the page changes via MutationObserver.

Known issue: while you are in homepage, superuser.com, every 1 minute the activity indicator which will show when new posts are asked or answered. Also, every relative timestamp, e.g. `answered 1 min ago` will become `answered 2 min ago`, and so on.

Well, while using the script, the latter feature, i.e. "the relative timestamps being increased every 1 min" becomes broken, and they don't get updated anymore.

The script modifies the values as follows (in my case, for timezone EEST, +0200) :

• tooltip: 2015-12-14 14:11:13Z --> 2015-12-14 16:11:13
• date text Dec 14 '15 at 14:11 --> Dec 14 '15 at 16:11

If I try to add a Z to the modified tooltips,
i.e. modify the line 65 to

dates[i].title = toTimeZone(temp, localTimezone) + 'Z';

therefore the text in the above case to become 2015-12-14 16:11:13Z
then the page hangs (the MutationObserver ).

The other approach I thought, was instead of modifying all tooltips+texts on page, to attach an event listener on each, to be activated on onmouseover and be restored on onmouseaway, but I think that (apart from filling the page with event listeners is not a good idea) the issue will remain, if at the point of "every 1 min", one tooltip has modified modified value at that time.

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*It uses the `jsTimezoneDetect` script (for getting the local timezone), and `Moment.js` and `Moment-Timezone` JavaScript libraries (for converting the dates). Also note: `jsTimezoneDetect` does not do geo-location, nor does it care very much about historical time zones, e.g. it may get "Europe/Berlin" when the user is in fact in "Europe/Stockholm" (they are both identical in modern time).*