Breaks many sites. After few minutes of debugging, I found out that its Date.getMonth() implementation is incorrect.

Mostly works; does not account for daylight savings time, and preferences page is awkward. I'd suggest a simple list of 10-12 zones to cover the majority of users, and an advanced list (in alphabetic order) for the rest.
Edit, after a few days of use: breaks a number of sites, including flight booking at westjet.com (calendars do not work at all) and managing my Mikrotik router (all dates are incorrect by a day.)

Полезное расширение, но очень не удобные настройки, выставлять Timezone пришлось вслепую , чтобы попасть в нужную. Указанные в настройках Timezone, не соответствуют реальным.
Спасибо за хорошее дополнение, нужно немного доработать, откалибровать.

Solves one problem but creates another. This plugin fixed my time issue with gmail but now pages that display calendars (such as picking hotels or airplane tickets) don't function properly. The calendar module of many different sites is just broken when this extension is in use. Fortunately it's easy enough to disable it when you need to purchase tickets and turn it off after but it is inconvenient to have to do so.

also fixed my time being always off due to privacy settings now i can have my time and not be confused

Uhm, Browserleaks.com still sees real system time via js toLocaleFormat and on browserspy.dk there is some additional info about date and time I not completely understand.
If this add-on doesn't mask all js date/time API then what's the point of using it?!

Fixed a problem I've had with Gmail for the longest time. It showed the wrong time for received emails.
It had something to do with about:config?filter=privacy.resistFingerprinting
This addon fixed my problem immediately.