TZ Environment variable is not considered

[JVApen]

In

date/src/tz.cpp

Line 4201 in 7875f43

tzdb::current_zone() const

, the linux code immediately reads the /etc/timezone file, while many programs support the TZ environment variable.

For example, if you execute the following:

TZ="Etc/UTC" date
TZ="Europe/Brussels" date

it results in:

Tue Jul 14 08:01:49 AM UTC 2026
Tue Jul 14 10:01:49 AM CEST 2026

One would expect similar behavior when using tzdb::current_zone()

[HowardHinnant]

This is by design. The use of the TZ global has multithreading issues. This library purposefully lacks a way to set the computer's timezone, such as via setenv. Instead this library is designed such that one can easily work with any time zone, simultaneously in any thread, and the computer's time zone is not special. It simply has the name current_zone().

You can easily work with any IANA time zone you want simply by using the locate_zone() function. You can even get the string from getenv("TZ") and use that as an argument to locate_zone() (or as an argument to zoned_time).

The logic of what happens if TZ doesn't exist, or contains a non-IANA name is entirely up to you. You could fall back to current_zone(), "Etc/UTC", or "Europe/London". And you'll have to deal with any multithreading issues yourself (e.g. if one thread sets TZ while another is reading it).

If your TZ contains a POSIX timezone string, you could utilize the "ptz.h" header found in this library if desired.

This library is now obsolete. It has been superseded in C++20 <chrono>. As you port to C++20 in the future, this behavior will remain consistent.