@wizeman, thanks for your PR! By analyzing the history of the files in this pull request, we identified @edolstra, @cpages and @abbradar to be potential reviewers.

Please wait just a few minutes as I'm about to push a better version of this PR.

Ok, with this updated version, systemd services are now restarted if the time zone changes.

I don't know if it still makes sense to keep the TZDIR variable now that TZ is defined. I was hoping that someone who better understands the implications of removing TZDIR would comment.

Ah, I believe TZDIR still needs to be set, because someone (either in their user environment or in a systemd service) may override TZ to be something like Europe/London, and in that case glibc must able to locate the time zone data correctly in NixOS, rather than assume the time zone data is in /usr/share (the default). This is issue #2447.

So I think we should keep the TZDIR definition (which this PR currently does).

Anything blocking this from either an in-extremis merge for 18.09 or an early merge for 19.03?

I'm don't have the required knowledge about glibc to confirm the speed assertions, neither the setup to test, but reading the appropriate manual section confirms this approach seems implemented right

The third format looks like this:

• :characters

Each operating system interprets this format differently; in the GNU C Library, characters is the name of a file which describes the time zone.

If the TZ environment variable does not have a value, the operation chooses a time zone by default. In the GNU C Library, the default time zone is like the specification ‘TZ=:/etc/localtime’ (or ‘TZ=:/usr/local/etc/localtime’, depending on how the GNU C Library was configured; see Installation). Other C libraries use their own rule for choosing the default time zone, so there is little we can say about them.

Looks good to me.

Rebased locally to test.

It breaks the timezone-imperative test, which works on the previous commit (current nixpkgs-unstable).

error: cannot coerce null to a string, at /.../nixpkgs-PR23078/nixos/modules/config/timezone.nix:12:15
(use '--show-trace' to show detailed location information)

This comes from the previous change where timeZone now default to null instead of "UTC".

I have added a WIP commit I'd like reviewed; the tests pass, and it seems semantically correct, but I'm not entirely confident (but pretty sure) it implements the change while not breaking the current assumptions.

cc @wizeman @vcunat @edolstra

@samueldr: Looks good to me!

Pushed on @wizeman's repo after the approval; fixing the merge conflict, now ready to merge.

(triage) @wizeman @samueldr anyone willing to fix the merge conflict?

Are there any updates on this pull request, please?

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I think I must have force-pushed my git branch by mistake and undid @samueldr's merge conflict fixes...

Note that while I was using these changes, I was running into a bug where Go programs couldn't parse the TZ=:/etc/localtime environment variable correctly and defaulted to the UTC timezone.

Perhaps golang/go#27570 would fix that (but I'm not sure).

I'm don't think we can do this, at least until this isn't supported in major language runtimes (like Go).

The commit golang/go@58fe2cd was landed, so Go 1.16+ will be ready for this change.

Cool! Did someone check if python supports this, too?

Edit: It seems at least pytz does.

@flokli It appearently works with Python 3.7 on NixOS 20.03:

[nix-shell:~]$ ls -l /etc/localtime
lrwxrwxrwx 1 root root 24 Oct 12 06:33 /etc/localtime -> /etc/zoneinfo/Asia/Tokyo

[nix-shell:~]$ TZ=UTC python
Python 3.7.6 (default, Dec 18 2019, 19:23:55)
[GCC 9.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> import time
>>> os.environ['TZ']
'UTC'
>>> time.tzset()
>>> time.tzname
('UTC', 'UTC')
>>> time.strftime('%X %x %Z')
'12:08:22 10/12/20 UTC'
>>> os.environ['TZ'] = ':/etc/localtime'
>>> time.tzset()
>>> time.tzname
('JST', 'JST')
>>> time.strftime('%X %x %Z')
'21:08:45 10/12/20 JST'
>>>

(JST: Japan Standard Time; same as Asia/Tokyo and its offset from UTC is +09:00)

@wizeman Can you rebase this?

@wizeman Are you still interested in pushing this through the last mile? :)

Here is a good news: Go 1.16 is released 🎉
I can take over if @wizeman @samueldr won't have enough time to work on this PR in near future.

@Ekleog I'm interested but I don't have enough time at the moment.

@omasanori Please, feel free to take over. Thanks!

@wizeman I see, Thank you for prompt response!

newer version in #113555