#### 22.5.6 Specifying the Time Zone with `TZ` ¶

In POSIX systems, a user can specify the time zone by means of the `TZ` environment variable. For information about how to set environment variables, see Environment Variables. The functions for accessing the time zone are declared in `time.h`.

You should not normally need to set `TZ`. If the system is configured properly, the default time zone will be correct. You might set `TZ` if you are using a computer over a network from a different time zone, and would like times reported to you in the time zone local to you, rather than what is local to the computer.

The value of `TZ` can be in one of the following formats:

- The geographical format specifies a location that stands for the past and future time zones observed in that location. See Geographical Format for `TZ`. Here are some examples:

      Asia/Tokyo
      America/New_York
      /usr/share/zoneinfo/America/Nuuk

- The proleptic format represents a time zone that has always been and always will be the same offset from UTC, optionally with a simple daylight saving scheme that has always been (and always will be) used every year. See Proleptic Format for `TZ`. Here are some examples:

      JST-9
      EST+5EDT,M3.2.0/2,M11.1.0/2
      <-02>+2<-01>,M3.5.0/-1,M10.5.0/0

- The colon format begins with ‘`:`’. Here is an example.

      :/etc/localtime

  Each operating system can interpret this format differently; in the GNU C Library, the ‘`:`’ is ignored and characters are treated as if they specified the geographical or proleptic format.

- As an extension to POSIX, when the value of `TZ` is the empty string, the GNU C Library uses UTC.

If the `TZ` environment variable does not have a value, the implementation 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 Installing the GNU C Library). Other C libraries use their own rule for choosing the default time zone, so there is little we can say about them.