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SCCS-file: Theory
SCCS-SID: 7.5
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Arthur David Olson authored and eggert committed Jul 18, 2012
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%W%

These time and date functions are much like the System V Release 2.0 (SVR2)
time and date functions; there are a few additions and changes to extend
the usefulness of the SVR2 functions:

* In SVR2, time display in a process is controlled by the environment
variable TZ, which "must be a three-letter time zone name, followed
by a number representing the difference between local time and
Greenwich Mean Time in hours, followed by an optional three-letter
name for a daylight time zone;" when the optional daylight time zone is
present, "standard U.S.A. Daylight Savings Time conversion is applied."
This means that SVR2 can't deal with other (for example, Australian)
daylight savings time rules, or situations where more than two

----- Outline -----

Time and date functions
Names of time zone regions
Time zone abbreviations


----- Time and date functions -----

These time and date functions are upwards compatible with POSIX.1,
an international standard for Unix-like systems.
As of this writing, the current edition of POSIX.1 is:

Information technology --Portable Operating System Interface (POSIX (R))
-- Part 1: System Application Program Interface (API) [C Language]
ISO/IEC 9945-1:1996
ANSI/IEEE Std 1003.1, 1996 Edition
1996-07-12

POSIX.1 has the following properties and limitations.

* In POSIX.1, time display in a process is controlled by the
environment variable TZ. Unfortunately, the POSIX.1 TZ string takes
a form that is hard to describe and is error-prone in practice.
Also, POSIX.1 TZ strings can't deal with other (for example, Israeli)
daylight saving time rules, or situations where more than two
time zone abbreviations are used in an area.

* In SVR2, time conversion information is compiled into each program
that does time conversion. This means that when time conversion
The POSIX.1 TZ string takes the following form:

stdoffset[dst[offset],date[/time],date[/time]]

where:

std and dst
are 3 or more characters specifying the standard
and daylight saving time (DST) zone names.
offset
is of the form `[-]hh:[mm[:ss]]' and specifies the
offset west of UTC. The default DST offset is one hour
ahead of standard time.
date[/time],date[/time]
specifies the beginning and end of DST. If this is absent,
the system supplies its own rules for DST, and these can
differ from year to year; typically US DST rules are used.
time
takes the form `hh:[mm[:ss]]' and defaults to 02:00.
date
takes one of the following forms:
Jn (1<=n<=365)
origin-1 day number not counting February 29
n (0<=n<=365)
origin-0 day number counting February 29 if present
Mm.n.d (0[Sunday]<=d<=6[Saturday], 1<=n<=5, 1<=m<=12)
for the dth day of week n of month m of the year,
where week 1 is the first week in which day d appears,
and `5' stands for the last week in which day d appears
(which may be either the 4th or 5th week).

* In POSIX.1, when a TZ value like "EST5EDT" is parsed,
typically the current US DST rules are used,
but this means that the US DST rules are compiled into each program
that does time conversion. This means that when US time conversion
rules change (as in the United States in 1987), all programs that
do time conversion must be recompiled to ensure proper results.

* In SVR2, time conversion fails for near-minimum or near-maximum
time_t values when doing conversions for places that don't use GMT.

* In SVR2, there's no tamper-proof way for a process to learn the
* In POSIX.1, there's no tamper-proof way for a process to learn the
system's best idea of local wall clock. (This is important for
applications that an administrator wants used only at certain times--
without regard to whether the user has fiddled the "TZ" environment
variable. While an administrator can "do everything in GMT" to get
around the problem, doing so is inconvenient and precludes handling
daylight savings time shifts--as might be required to limit phone
daylight saving time shifts--as might be required to limit phone
calls to off-peak hours.)

* These functions can account for leap seconds, thanks to Bradley White
(bww@k.cs.cmu.edu).
* POSIX.1 requires that systems ignore leap seconds.

These are the changes that have been made to the SVR2 functions:
These are the extensions that have been made to the POSIX.1 functions:

* The "TZ" environment variable is used in generating the name of a file
from which time zone information is read (or is interpreted a la
Expand Down Expand Up @@ -62,13 +106,12 @@ These are the changes that have been made to the SVR2 functions:
* To handle places where more than two time zone abbreviations are used,
the functions "localtime" and "gmtime" set tzname[tmp->tm_isdst]
(where "tmp" is the value the function returns) to the time zone
abbreviation to be used. This differs from SVR2, where the elements
abbreviation to be used. This differs from POSIX.1, where the elements
of tzname are only changed as a result of calls to tzset.

* Since the "TZ" environment variable can now be used to control time
conversion, the "daylight" and "timezone" variables are no longer
needed or supported. (You can use a compile-time option to cause
these variables to be defined and to be set by "tzset"; however, their
needed. (These variables are defined and set by "tzset"; however, their
values will not be used by "localtime.")

* The "localtime" function has been set up to deliver correct results
Expand All @@ -86,9 +129,21 @@ These are the changes that have been made to the SVR2 functions:
environment variable; portable applications should not, however, rely
on this behavior since it's not the way SVR2 systems behave.)

Points of interest to folks with Version 7 or BSD systems:
* These functions can account for leap seconds, thanks to Bradley White
(bww@k.cs.cmu.edu).

Points of interest to folks with other systems:

* The BSD "timezone" function is not present in this package;
* This package is already part of many POSIX-compliant hosts,
including BSD, HP, Linux, Network Appliance, SCO, SGI, and Sun.
On such hosts, the primary use of this package
is to update obsolete time zone rule tables.
To do this, you may need to compile the time zone compiler
`zic' supplied with this package instead of using the system `zic',
since the format of zic's input changed slightly in late 1994,
and many vendors still do not support the new input format.

* The Unix Version 7 "timezone" function is not present in this package;
it's impossible to reliably map timezone's arguments (a "minutes west
of GMT" value and a "daylight saving time in effect" flag) to a
time zone abbreviation, and we refuse to guess.
Expand All @@ -97,12 +152,13 @@ Points of interest to folks with Version 7 or BSD systems:
zone abbreviation to use. Alternatively, use
localtime(&clock)->tm_zone if this has been enabled.

* The BSD gettimeofday function is not used in this package;
this lets users control the time zone used in doing time conversions.
Users who don't try to control things (that is, users who do not set
the environment variable TZ) get the time conversion specified in the
file "/etc/zoneinfo/localtime"; see the time zone compiler writeup for
information on how to initialize this file.
* The 4.2BSD gettimeofday function is not used in this package.
This formerly let users obtain the current UTC offset and DST flag,
but this functionality was removed in later versions of BSD.

* In SVR2, time conversion fails for near-minimum or near-maximum
time_t values when doing conversions for places that don't use GMT.
This package takes care to do these conversions correctly.

The functions that are conditionally compiled if STD_INSPIRED is defined
should, at this point, be looked on primarily as food for thought. They are
Expand All @@ -115,6 +171,115 @@ Hewlett Packard, offer a wider selection of functions that provide capabilities
beyond those provided here. The absence of such functions from this package
is not meant to discourage the development, standardization, or use of such
functions. Rather, their absence reflects the decision to make this package
close to SVR2 (with the exceptions outlined above) to ensure its broad
contain valid extensions to POSIX.1, to ensure its broad
acceptability. If more powerful time conversion functions can be standardized,
so much the better.


----- Names of time zone rule files -----

The names of this package's installed time zone rule files are chosen to
help minimize possible future incompatibilities due to political events.
Ordinarily, names of countries are not used, to avoid incompatibilities
when countries change their name (e.g. Zaire->Congo) or
when locations change countries (e.g. Hong Kong from UK colony to China).

Names normally have the form AREA/LOCATION, where AREA is the name
of a continent or ocean, and LOCATION is the name of a specific
location within that region. North and South America share the same
area, `America'. Typical names are `Africa/Cairo', `America/New_York',
and `Pacific/Honolulu'.

Here are the general rules used for choosing location names,
in decreasing order of importance:

Use only valid Posix file names. Use only Ascii letters, digits, `.',
`-' and `_'. Do not exceed 14 characters or start with `-'.
E.g. prefer `Brunei' to `Bandar_Seri_Begawan'.
Include at least one location per time zone rule set per country.
One such location is enough.
If all the clocks in a country's region have agreed since 1970,
don't bother to include more than one location
even if subregions' clocks disagreed before 1970.
Otherwise these tables would become annoyingly large.
If a name is ambiguous, use a less ambiguous alternative;
e.g. many cities are named San Jose and Georgetown, so
prefer `Costa_Rica' to `San_Jose' and `Guyana' to `Georgetown'.
Keep locations compact. Use cities or small islands, not countries
or regions, so that any future time zone changes do not split
locations into different time zones. E.g. prefer `Paris'
to `France', since France has had multiple time zones.
Use traditional English spelling, e.g. prefer `Rome' to `Roma', and
prefer `Athens' to the true name (which uses Greek letters).
The Posix file name restrictions encourage this rule.
Use the most populous among locations in a country's time zone,
e.g. prefer `Shanghai' to `Beijing'. Among locations with
similar populations, pick the best-known location,
e.g. prefer `Rome' to `Milan'.
Use the singular form, e.g. prefer `Canary' to `Canaries'.
Omit common suffixes like `_Islands' and `_City', unless that
would lead to ambiguity. E.g. prefer `Cayman' to
`Cayman_Islands' and `Guatemala' to `Guatemala_City',
but prefer `Mexico_City' to `Mexico' because the country
of Mexico has several time zones.
Use `_' to represent a space.
Omit `.' from abbreviations in names, e.g. prefer `St_Helena'
to `St._Helena'.

The file `zone.tab' lists the geographical locations used to name
time zone rule files.

Older versions of this package used a different naming scheme,
and these older names are still supported.
See the file `backwards' for most of these older names
(e.g. `US/Eastern' instead of `America/New_York').
The other old-fashioned names still supported are
`WET', `CET', `MET', `EET' (see the file `europe'),
and `Factory' (see the file `factory').


----- Time zone abbreviations -----

When this package is installed, it generates time zone abbreviations
like `EST' to be compatible with human tradition and POSIX.1.
Here are the general rules used for choosing time zone abbreviations,
in decreasing order of importance:

Use abbreviations that consist of 3 or more upper-case Ascii letters,
except use "___" for locations while uninhabited.
Posix.1 requires at least 3 characters, and the restriction to
upper-case Ascii letters follows most traditions.
Previous editions of this database also used characters like
' ' and '?', but these characters have a special meaning to
the shell and cause commands like
set `date`
to have unexpected effects. In theory, the character set could
be !%./@A-Z^_a-z{}, but these tables use only upper-case
Ascii letters (and "___").
Use abbreviations that are in common use among English-speakers,
e.g. `EST' for Eastern Standard Time in North America.
We assume that applications translate them to other languages
as part of the normal localization process; for example,
a French application might translate `EST' to `HNE'.
For zones whose times are taken from a city's longitude, use the
traditional xMT notation, e.g. `PMT' for Paris Mean Time.
The only name like this in current use is `GMT'.
If there is no common English abbreviation, abbreviate the English
translation of the usual phrase used by native speakers.
If this is not available or is a phrase mentioning the country
(e.g. ``Cape Verde Time''), then:

When a country has a single or principal time zone region,
append `T' to the country's ISO code, e.g. `CVT' for
Cape Verde Time. For summer time append `ST';
for double summer time append `DST'; etc.
When a country has multiple time zones, take the first three
letters of an English place name identifying each zone
and then append `T', `ST', etc. as before;
e.g. `VLAST' for VLAdivostok Summer Time.

Application writers should note that these abbreviations are ambiguous
in practice: e.g. `EST' has a different meaning in Australia than
it does in the United States. In new applications, it's often better
to use numeric GMT offsets like `-0500' instead of time zone
abbreviations like `EST'; this avoids the ambiguity.

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