Instead of fixing the extension and making it really user-friendly, the author of the Donat button adds. Well, here's another one.

It doesn't set the time correctly. Apparently does not take into account the transfer of winter/summer time. Useless as an automaton, since every time after the time transfer, you have to go to a third-party site and check the offset.

There is no function to quickly call up settings. You need to go to add-ons >> add-on settings to change the time. And at the same time, there is an absolutely useless button on the panel. It’s strange.

Add a select Timezone in the icon menu on the panel. When I switch proxies / vpn I need to go to add-on management to switch Timezone. But then you put the check page in the icon. Don't you think it's a little through the ass? =) I myself know where to check the time, I need to switch it, and not look at the clock.

For example, 4-5 presets in the menu would suit me perfectly.

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.

Great Add-ons! I will no longer have to change my system timezone manually like before. :D

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.