1567862 - JS engine's `new Date().getTimezoneOffset()` is wrong if the browser was started in a different timezone to the current one

Status: RESOLVED DUPLICATE of bug 1348916

(Core :: JavaScript Engine, defect)

Description

[:Gijs (he/him)]

I traveled from Europe to the UK this weekend. My browser instance was started in Europe and is still alive. This is some browser console output:

> new Date().getTimezoneOffset()
< -120
> new Date().toString()
< "Mon Jul 22 2019 12:38:11 GMT+0200 (Central European Summer Time)"
> new Date()
< Date Mon Jul 22 2019 11:38:27 GMT+0100 (British Summer Time)
# note: I don't know how devtools does this, but it somehow manages to get it right...

This is a bit annoying. I don't know if a summer time change would cause the same issue, but it seems concerning. I'm not at all a JS engine expert, but from a casual check, the use of cachedLocalTime at https://searchfox.org/mozilla-central/rev/b9041f813de0a05bf6de95a145d4e25004499517/js/src/jsdate.cpp#1902,1906 looks like it might be the culprit. I guess in the ideal case we should update whatever it's caching when the OS notifies us of a timezone change (assuming that we're using caching for performance reasons). In case it matters, I'm on mac (fx69 beta).

Comment 1

[:Gijs (he/him)]

Oh, and:

> Intl.DateTimeFormat().resolvedOptions().timeZone 
< "Europe/Amsterdam"