If `TZ` is null, the best available approximation to local (wall clock) time, as
specified by the `tzfile`(5)-format file `localtime` in the system time
conversion information directory (typically `/etc`), is used.  If `TZ` is the
empty string, UT is used, with the abbreviation "UTC" and without leap second
correction; please see `newctime`(3) for more about UT, UTC, and leap seconds.
If `TZ` is nonnull and nonempty:

- if the value begins with a colon, it is used as a pathname of a file from
which to read the time conversion information;

- if the value does not begin with a colon, it is first used as the pathname of
a file from which to read the time conversion information, and, if that file
cannot be read, is used directly as a specification of the time conversion
information.

When `TZ` is used as a pathname, if it begins with a slash, it is used as an
absolute pathname; otherwise, it is used as a pathname relative to a system time
conversion information directory.  The file must be in the format specified in
`tzfile`(5).

When `TZ` is used directly as a specification of the time conversion
information, it must have the following syntax (spaces inserted for clarity):
```text
std offset[dst[offset][,rule]]
```

Where:

- `std` and `dst`

  Three or more bytes that are the designation for the standard (`std`) or the
  alternative (`dst`, such as daylight saving time) time zone.  Only `std` is
  required; if `dst` is missing, then daylight saving time does not apply in
  this locale.  Upper- and lowercase letters are explicitly allowed.  Any
  characters except a leading colon (`:`), digits, comma (`,`), ASCII minus (-),
  ASCII plus (`+`), and NUL bytes are allowed.  Alternatively, a designa tion
  can be surrounded by angle brackets `<` and `>`; in this case, the designation
  can contain any characters other than `>` and NUL.

- `offset`

  Indicates the value one must add to the local time to arrive at Coordinated
  Universal Time.  The `offset` has the form:

  ```text
  hh[:mm[:ss]]
  ```

  The minutes (`mm`) and seconds (`ss`) are optional.  The hour (`hh`) is
  required and may be a single digit.  The `offset` following `std` is required.
  If no `offset` follows `dst`, daylight saving time is assumed to be one hour
  ahead of standard time.  One or more digits may be used; the value is al ways
  interpreted as a decimal number.  The hour must be between zero and 24, and
  the minutes (and seconds) – if present – between zero and 59.  If preceded by
  a “-”, the time zone shall be east of the Prime Meridian; otherwise it shall
  be west (which may be indicated by an optional preceding “+”.

- `rule`

  Indicates when to change to and back from day light saving time.  The `rule`
  has the form:

  ```text
  date/time,date/time
  ```

  where the first `date` describes when the change from standard to daylight
  saving time occurs and the second `date` describes when the change back
  happens.  Each `time` field describes when, in cur rent local time, the change
  to the other time is made.  As an extension to POSIX, daylight saving is
  assumed to be in effect all year if it begins January 1 at 00:00 and ends
  December 31 at 24:00 plus the difference between daylight saving and standard
  time, leaving no room for standard time in the calendar.

  The format of `date` is one of the following:

  - `Jn`

    The Julian day `n` (1 ≤ `n` ≤ 365).  Leap days are not counted; that is, in
    all years – including leap years – February 28 is day 59 and March 1 is
    day 60.  It is impossible to explicitly refer to the occasional February 29.

  - `n`

    The zero-based Julian day (0 ≤ `n` ≤ 365).  Leap days are counted, and it is
    possible to refer to February 29.

  - `Mm.n.d`

    The `d'`th day (0 ≤ `d` ≤ 6) of week `n` of month `m` of the year (1 ≤ `n` ≤
    5, 1 ≤ `m` ≤ 12, where week 5 means “the last `d` day in month `m`” which
    may occur in either the fourth or the fifth week).  Week 1 is the first week
    in which the `d'`th day occurs.  Day zero is Sunday.

  The `time` has the same format as `offset` except that POSIX does not allow a
  leading sign (`-` or `+`).  As an extension to POSIX, the hours part of `time`
  can range from -167 through 167; this al lows for unusual rules such as “the
  Saturday be fore the first Sunday of March”.  The default, if `time` is not
  given, is `02:00:00`.

# Examples

Here are some examples of `TZ` values that directly specify the timezone; they
use some of the extensions to POSIX.

```text
EST5
```

stands for US Eastern Standard Time (EST), 5 hours behind UT, without daylight
saving.

```text
<+12>-12<+13>,M11.1.0,M1.2.1/147
```

stands for Fiji time, 12 hours ahead of UT, springing forward on November's
first Sunday at 02:00, and falling back on January's second Monday at 147:00
(i.e., 03:00 on the first Sunday on or after January 14).  The abbreviations for
standard and daylight saving time are “+12” and “+13”.

```text
IST-2IDT,M3.4.4/26,M10.5.0
```

stands for Israel Standard Time (IST) and Israel Daylight Time (IDT), 2 hours
ahead of UT, springing forward on March's fourth Thursday at 26:00 (i.e., 02:00
on the first Friday on or after March 23), and falling back on October's last
Sunday at 02:00.

```text
<-04>4<-03>,J1/0,J365/25
```

stands for permanent daylight saving time, 3 hours behind UT with abbreviation
“-03”.  There is a dummy fall-back transition on December 31 at 25:00 daylight
saving time (i.e., 24:00 standard time, equivalent to January 1 at 00:00
standard time), and a simultaneous spring-forward transition on January 1 at
00:00 standard time, so daylight saving time is in effect all year and the
initial `<-04>` is a placeholder.

```text
<-03>3<-02>,M3.5.0/-2,M10.5.0/-1
```

stands for time in western Greenland, 3 hours behind UT, where clocks follow the
EU rules of springing forward on March's last Sunday at 01:00 UT (-02:00 local
time, i.e., 22:00 the previous day) and falling back on October's last Sunday at
01:00 UT (-01:00 local time, i.e., 23:00 the previous day).  The abbreviations
for standard and daylight saving time are `-03` and `-02`.

If no `rule` is present in `TZ`, the rules specified by the `tzfile`(5)-format
file `posixrules` in the system time conversion information directory are used,
with the standard and daylight saving time offsets from UT replaced by those
specified by the `offset` values in `TZ`.

For compatibility with System V Release 3.1, a  semicolon  (`;`)  may  be
used to separate the `rule` from the rest of the specification.

# Files
| Path                             | Description                    |
|----------------------------------|--------------------------------|
| `/usr/share/zoneinfo`            | timezone information directory |
| `/usr/share/zoneinfo/localtime`  | local timezone file            |
| `/usr/share/zoneinfo/posixrules` | used with POSIX-style TZ       |
| `/usr/share/zoneinfo/GMT`        | for UTC leap seconds           |

If `/usr/share/zoneinfo/GMT` is absent, UTC leap seconds are loaded from
`/usr/share/zoneinfo/posixrules`.