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<!DOCTYPE html>
<html lang="en">
<head>
<title>Theory and pragmatics of the tz code and data</title>
<meta charset="UTF-8">
<style>
pre {margin-left: 2em; white-space: pre-wrap;}
</style>
</head>
<body>
<h1>Theory and pragmatics of the <code><abbr>tz</abbr></code> code and data</h1>
<h3>Outline</h3>
<nav>
<ul>
<li><a href="#scope">Scope of the <code><abbr>tz</abbr></code>
database</a></li>
<li><a href="#naming">Timezone identifiers</a></li>
<li><a href="#abbreviations">Time zone abbreviations</a></li>
<li><a href="#accuracy">Accuracy of the <code><abbr>tz</abbr></code>
database</a></li>
<li><a href="#functions">Time and date functions</a></li>
<li><a href="#stability">Interface stability</a></li>
<li><a href="#leapsec">Leap seconds</a></li>
<li><a href="#calendar">Calendrical issues</a></li>
<li><a href="#planets">Time and time zones on other planets</a></li>
</ul>
</nav>
<section>
<h2 id="scope">Scope of the <code><abbr>tz</abbr></code> database</h2>
<p>
The <a
href="https://www.iana.org/time-zones"><code><abbr>tz</abbr></code>
database</a> attempts to record the history and predicted future of
civil time scales.
It organizes <a href="tz-link.html">time zone and daylight saving time
data</a> by partitioning the world into <a
href="https://en.wikipedia.org/wiki/List_of_tz_database_time_zones"><dfn>timezones</dfn></a>
whose clocks all agree about timestamps that occur after the <a
href="https://en.wikipedia.org/wiki/Unix_time">POSIX Epoch</a>
(1970-01-01 00:00:00 <a
href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time"><abbr
title="Coordinated Universal Time">UTC</abbr></a>).
Although 1970 is a somewhat-arbitrary cutoff, there are significant
challenges to moving the cutoff earlier even by a decade or two, due
to the wide variety of local practices before computer timekeeping
became prevalent.
Most timezones correspond to a notable location and the database
records all known clock transitions for that location;
some timezones correspond instead to a fixed <abbr>UTC</abbr> offset.
</p>
<p>
Each timezone typically corresponds to a geographical region that is
smaller than a traditional time zone, because clocks in a timezone
all agree after 1970 whereas a traditional time zone merely
specifies current standard time. For example, applications that deal
with current and future timestamps in the traditional North
American mountain time zone can choose from the timezones
<code>America/Denver</code> which observes US-style daylight saving
time (<abbr>DST</abbr>),
<code>America/Mazatlan</code> which observes Mexican-style <abbr>DST</abbr>,
and <code>America/Phoenix</code> which does not observe <abbr>DST</abbr>.
Applications that also deal with past timestamps in the mountain time
zone can choose from over a dozen timezones, such as
<code>America/Boise</code>, <code>America/Edmonton</code>, and
<code>America/Hermosillo</code>, each of which currently uses mountain
time but differs from other timezones for some timestamps after 1970.
</p>
<p>
Clock transitions before 1970 are recorded for location-based timezones,
because most systems support timestamps before 1970 and could
misbehave if data entries were omitted for pre-1970 transitions.
However, the database is not designed for and does not suffice for
applications requiring accurate handling of all past times everywhere,
as it would take far too much effort and guesswork to record all
details of pre-1970 civil timekeeping.
Although some information outside the scope of the database is
collected in a file <code>backzone</code> that is distributed along
with the database proper, this file is less reliable and does not
necessarily follow database guidelines.
</p>
<p>
As described below, reference source code for using the
<code><abbr>tz</abbr></code> database is also available.
The <code><abbr>tz</abbr></code> code is upwards compatible with <a
href="https://en.wikipedia.org/wiki/POSIX">POSIX</a>, an international
standard for <a
href="https://en.wikipedia.org/wiki/Unix">UNIX</a>-like systems.
As of this writing, the current edition of POSIX is: <a
href="https://pubs.opengroup.org/onlinepubs/9699919799/"> The Open
Group Base Specifications Issue 7</a>, IEEE Std 1003.1-2017, 2018
Edition.
Because the database's scope encompasses real-world changes to civil
timekeeping, its model for describing time is more complex than the
standard and daylight saving times supported by POSIX.
A <code><abbr>tz</abbr></code> timezone corresponds to a ruleset that can
have more than two changes per year, these changes need not merely
flip back and forth between two alternatives, and the rules themselves
can change at times.
Whether and when a timezone changes its clock,
and even the timezone's notional base offset from <abbr>UTC</abbr>,
are variable.
It does not always make sense to talk about a timezone's
"base offset", which is not necessarily a single number.
</p>
</section>
<section>
<h2 id="naming">Timezone identifiers</h2>
<p>
Each timezone has a name that uniquely identifies the timezone.
Inexperienced users are not expected to select these names unaided.
Distributors should provide documentation and/or a simple selection
interface that explains each name via a map or via descriptive text like
"Czech Republic" instead of the timezone name "<code>Europe/Prague</code>".
If geolocation information is available, a selection interface can
locate the user on a timezone map or prioritize names that are
geographically close. For an example selection interface, see the
<code>tzselect</code> program in the <code><abbr>tz</abbr></code> code.
The <a href="http://cldr.unicode.org">Unicode Common Locale Data
Repository</a> contains data that may be useful for other selection
interfaces; it maps timezone names like <code>Europe/Prague</code> to
locale-dependent strings like "Prague", "Praha", "Прага", and "布拉格".
</p>
<p>
The naming conventions attempt to strike a balance
among the following goals:
</p>
<ul>
<li>
Uniquely identify every timezone where clocks have agreed since 1970.
This is essential for the intended use: static clocks keeping local
civil time.
</li>
<li>
Indicate to experts where the timezone's clocks typically are.
</li>
<li>
Be robust in the presence of political changes.
For example, names are typically not tied to countries, to avoid
incompatibilities when countries change their name (e.g.,
Swaziland→Eswatini) or when locations change countries (e.g., Hong
Kong from UK colony to China).
There is no requirement that every country or national
capital must have a timezone name.
</li>
<li>
Be portable to a wide variety of implementations.
</li>
<li>
Use a consistent naming conventions over the entire world.
</li>
</ul>
<p>
Names normally have the form
<var>AREA</var><code>/</code><var>LOCATION</var>, where
<var>AREA</var> is a continent or ocean, and
<var>LOCATION</var> is a specific location within the area.
North and South America share the same area, '<code>America</code>'.
Typical names are '<code>Africa/Cairo</code>',
'<code>America/New_York</code>', and '<code>Pacific/Honolulu</code>'.
Some names are further qualified to help avoid confusion; for example,
'<code>America/Indiana/Petersburg</code>' distinguishes Petersburg,
Indiana from other Petersburgs in America.
</p>
<p>
Here are the general guidelines used for
choosing timezone names,
in decreasing order of importance:
</p>
<ul>
<li>
Use only valid POSIX file name components (i.e., the parts of
names other than '<code>/</code>').
Do not use the file name components '<code>.</code>' and
'<code>..</code>'.
Within a file name component, use only <a
href="https://en.wikipedia.org/wiki/ASCII">ASCII</a> letters,
'<code>.</code>', '<code>-</code>' and '<code>_</code>'.
Do not use digits, as that might create an ambiguity with <a
href="https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03">POSIX
<code>TZ</code> strings</a>.
A file name component must not exceed 14 characters or start with
'<code>-</code>'.
E.g., prefer <code>America/Noronha</code> to
<code>America/Fernando_de_Noronha</code>.
Exceptions: see the discussion of legacy names below.
</li>
<li>
A name must not be empty, or contain '<code>//</code>', or
start or end with '<code>/</code>'.
</li>
<li>
Do not use names that differ only in case.
Although the reference implementation is case-sensitive, some
other implementations are not, and they would mishandle names
differing only in case.
</li>
<li>
If one name <var>A</var> is an initial prefix of another
name <var>AB</var> (ignoring case), then <var>B</var> must not
start with '<code>/</code>', as a regular file cannot have the
same name as a directory in POSIX.
For example, <code>America/New_York</code> precludes
<code>America/New_York/Bronx</code>.
</li>
<li>
Uninhabited regions like the North Pole and Bouvet Island
do not need locations, since local time is not defined there.
</li>
<li>
If all the clocks in a timezone have agreed since 1970,
do not bother to include more than one timezone
even if some of the clocks disagreed before 1970.
Otherwise these tables would become annoyingly large.
</li>
<li>
If boundaries between regions are fluid, such as during a war or
insurrection, do not bother to create a new timezone merely
because of yet another boundary change. This helps prevent table
bloat and simplifies maintenance.
</li>
<li>
If a name is ambiguous, use a less ambiguous alternative;
e.g., many cities are named San José and Georgetown, so
prefer <code>America/Costa_Rica</code> to
<code>America/San_Jose</code> and <code>America/Guyana</code>
to <code>America/Georgetown</code>.
</li>
<li>
Keep locations compact.
Use cities or small islands, not countries or regions, so that any
future changes do not split individual locations into different
timezones.
E.g., prefer <code>Europe/Paris</code> to <code>Europe/France</code>,
since
<a href="https://en.wikipedia.org/wiki/Time_in_France#History">France
has had multiple time zones</a>.
</li>
<li>
Use mainstream English spelling, e.g., prefer
<code>Europe/Rome</code> to <code>Europa/Roma</code>, and
prefer <code>Europe/Athens</code> to the Greek
<code>Ευρώπη/Αθήνα</code> or the Romanized
<code>Evrópi/Athína</code>.
The POSIX file name restrictions encourage this guideline.
</li>
<li>
Use the most populous among locations in a region,
e.g., prefer <code>Asia/Shanghai</code> to
<code>Asia/Beijing</code>.
Among locations with similar populations, pick the best-known
location, e.g., prefer <code>Europe/Rome</code> to
<code>Europe/Milan</code>.
</li>
<li>
Use the singular form, e.g., prefer <code>Atlantic/Canary</code> to
<code>Atlantic/Canaries</code>.
</li>
<li>
Omit common suffixes like '<code>_Islands</code>' and
'<code>_City</code>', unless that would lead to ambiguity.
E.g., prefer <code>America/Cayman</code> to
<code>America/Cayman_Islands</code> and
<code>America/Guatemala</code> to
<code>America/Guatemala_City</code>, but prefer
<code>America/Mexico_City</code> to
<code>America/Mexico</code>
because <a href="https://en.wikipedia.org/wiki/Time_in_Mexico">the
country of Mexico has several time zones</a>.
</li>
<li>
Use '<code>_</code>' to represent a space.
</li>
<li>
Omit '<code>.</code>' from abbreviations in names.
E.g., prefer <code>Atlantic/St_Helena</code> to
<code>Atlantic/St._Helena</code>.
</li>
<li>
Do not change established names if they only marginally violate
the above guidelines.
For example, do not change the existing name <code>Europe/Rome</code> to
<code>Europe/Milan</code> merely because Milan's population has grown
to be somewhat greater than Rome's.
</li>
<li>
If a name is changed, put its old spelling in the
'<code>backward</code>' file.
This means old spellings will continue to work.
Ordinarily a name change should occur only in the rare case when
a location's consensus English-language spelling changes; for example,
in 2008 <code>Asia/Calcutta</code> was renamed to <code>Asia/Kolkata</code>
due to long-time widespread use of the new city name instead of the old.
</li>
</ul>
<p>
Guidelines have evolved with time, and names following old versions of
these guidelines might not follow the current version. When guidelines
have changed, old names continue to be supported. Guideline changes
have included the following:
</p>
<ul>
<li>
Older versions of this package used a different naming scheme.
See the file '<code>backward</code>' for most of these older names
(e.g., '<code>US/Eastern</code>' instead of '<code>America/New_York</code>').
The other old-fashioned names still supported are
'<code>WET</code>', '<code>CET</code>', '<code>MET</code>', and
'<code>EET</code>' (see the file '<code>europe</code>').
</li>
<li>
Older versions of this package defined legacy names that are
incompatible with the first guideline of location names, but which are
still supported.
These legacy names are mostly defined in the file
'<code>etcetera</code>'.
Also, the file '<code>backward</code>' defines the legacy names
'<code>GMT0</code>', '<code>GMT-0</code>' and '<code>GMT+0</code>',
and the file '<code>northamerica</code>' defines the legacy names
'<code>EST5EDT</code>', '<code>CST6CDT</code>',
'<code>MST7MDT</code>', and '<code>PST8PDT</code>'.
</li>
<li>
Older versions of these guidelines said that
there should typically be at least one name for each <a
href="https://en.wikipedia.org/wiki/ISO_3166-1"><abbr
title="International Organization for Standardization">ISO</abbr>
3166-1</a> officially assigned two-letter code for an inhabited
country or territory.
This old guideline has been dropped, as it was not needed to handle
timestamps correctly and it increased maintenance burden.
</li>
</ul>
<p>
The file '<code>zone1970.tab</code>' lists geographical locations used
to name timezones.
It is intended to be an exhaustive list of names for geographic
regions as described above; this is a subset of the timezones in the data.
Although a '<code>zone1970.tab</code>' location's
<a href="https://en.wikipedia.org/wiki/Longitude">longitude</a>
corresponds to
its <a href="https://en.wikipedia.org/wiki/Local_mean_time">local mean
time (<abbr>LMT</abbr>)</a> offset with one hour for every 15°
east longitude, this relationship is not exact.
</p>
<p>
Excluding '<code>backward</code>' should not affect the other data.
If '<code>backward</code>' is excluded, excluding
'<code>etcetera</code>' should not affect the remaining data.
</p>
</section>
<section>
<h2 id="abbreviations">Time zone abbreviations</h2>
<p>
When this package is installed, it generates time zone abbreviations
like '<code>EST</code>' to be compatible with human tradition and POSIX.
Here are the general guidelines used for choosing time zone abbreviations,
in decreasing order of importance:
</p>
<ul>
<li>
Use three to six characters that are ASCII alphanumerics or
'<code>+</code>' or '<code>-</code>'.
Previous editions of this database also used characters like
space and '<code>?</code>', but these characters have a
special meaning to the
<a href="https://en.wikipedia.org/wiki/Unix_shell">UNIX shell</a>
and cause commands like
'<code><a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#set">set</a>
`<a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/date.html">date</a>`</code>'
to have unexpected effects.
Previous editions of this guideline required upper-case letters, but the
Congressman who introduced
<a href="https://en.wikipedia.org/wiki/Chamorro_Time_Zone">Chamorro
Standard Time</a> preferred "ChST", so lower-case letters are now
allowed.
Also, POSIX from 2001 on relaxed the rule to allow '<code>-</code>',
'<code>+</code>', and alphanumeric characters from the portable
character set in the current locale.
In practice ASCII alphanumerics and '<code>+</code>' and
'<code>-</code>' are safe in all locales.
<p>
In other words, in the C locale the POSIX extended regular
expression <code>[-+[:alnum:]]{3,6}</code> should match the
abbreviation.
This guarantees that all abbreviations could have been specified by a
POSIX <code>TZ</code> string.
</p>
</li>
<li>
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'.
<p>
<small>These abbreviations (for standard/daylight/etc. time) are:
ACST/ACDT Australian Central,
AST/ADT/APT/AWT/ADDT Atlantic,
AEST/AEDT Australian Eastern,
AHST/AHDT Alaska-Hawaii,
AKST/AKDT Alaska,
AWST/AWDT Australian Western,
BST/BDT Bering,
CAT/CAST Central Africa,
CET/CEST/CEMT Central European,
ChST Chamorro,
CST/CDT/CWT/CPT/CDDT Central [North America],
CST/CDT China,
GMT/BST/IST/BDST Greenwich,
EAT East Africa,
EST/EDT/EWT/EPT/EDDT Eastern [North America],
EET/EEST Eastern European,
GST/GDT Guam,
HST/HDT/HWT/HPT Hawaii,
HKT/HKST/HKWT Hong Kong,
IST India,
IST/GMT Irish,
IST/IDT/IDDT Israel,
JST/JDT Japan,
KST/KDT Korea,
MET/MEST Middle European (a backward-compatibility alias for
Central European),
MSK/MSD Moscow,
MST/MDT/MWT/MPT/MDDT Mountain,
NST/NDT/NWT/NPT/NDDT Newfoundland,
NST/NDT/NWT/NPT Nome,
NZMT/NZST New Zealand through 1945,
NZST/NZDT New Zealand 1946–present,
PKT/PKST Pakistan,
PST/PDT/PWT/PPT/PDDT Pacific,
PST/PDT Philippine,
SAST South Africa,
SST Samoa,
WAT/WAST West Africa,
WET/WEST/WEMT Western European,
WIB Waktu Indonesia Barat,
WIT Waktu Indonesia Timur,
WITA Waktu Indonesia Tengah,
YST/YDT/YWT/YPT/YDDT Yukon</small>.
</p>
</li>
<li>
<p>
For times taken from a city's longitude, use the
traditional <var>x</var>MT notation.
The only abbreviation like this in current use is '<abbr>GMT</abbr>'.
The others are for timestamps before 1960,
except that Monrovia Mean Time persisted until 1972.
Typically, numeric abbreviations (e.g., '<code>-</code>004430' for
MMT) would cause trouble here, as the numeric strings would exceed
the POSIX length limit.
</p>
<p>
<small>These abbreviations are:
AMT Amsterdam, Asunción, Athens;
BMT Baghdad, Bangkok, Batavia, Bermuda, Bern, Bogotá, Bridgetown,
Brussels, Bucharest;
CMT Calamarca, Caracas, Chisinau, Colón, Copenhagen, Córdoba;
DMT Dublin/Dunsink;
EMT Easter;
FFMT Fort-de-France;
FMT Funchal;
GMT Greenwich;
HMT Havana, Helsinki, Horta, Howrah;
IMT Irkutsk, Istanbul;
JMT Jerusalem;
KMT Kaunas, Kiev, Kingston;
LMT Lima, Lisbon, local, Luanda;
MMT Macassar, Madras, Malé, Managua, Minsk, Monrovia, Montevideo,
Moratuwa, Moscow;
PLMT Phù Liễn;
PMT Paramaribo, Paris, Perm, Pontianak, Prague;
PMMT Port Moresby;
QMT Quito;
RMT Rangoon, Riga, Rome;
SDMT Santo Domingo;
SJMT San José;
SMT Santiago, Simferopol, Singapore, Stanley;
TBMT Tbilisi;
TMT Tallinn, Tehran;
WMT Warsaw;
ZMT Zomba.</small>
</p>
<p>
<small>A few abbreviations also follow the pattern that
<abbr>GMT</abbr>/<abbr>BST</abbr> established for time in the UK.
They are:
BMT/BST for Bermuda 1890–1930,
CMT/BST for Calamarca Mean Time and Bolivian Summer Time
1890–1932,
DMT/IST for Dublin/Dunsink Mean Time and Irish Summer Time
1880–1916,
MMT/MST/MDST for Moscow 1880–1919, and
RMT/LST for Riga Mean Time and Latvian Summer time 1880–1926.
An extra-special case is SET for Swedish Time (<em>svensk
normaltid</em>) 1879–1899, 3° west of the Stockholm
Observatory.</small>
</p>
</li>
<li>
Use '<abbr>LMT</abbr>' for local mean time of locations before the
introduction of standard time; see "<a href="#scope">Scope of the
<code><abbr>tz</abbr></code> database</a>".
</li>
<li>
If there is no common English abbreviation, use numeric offsets like
<code>-</code>05 and <code>+</code>0530 that are generated
by <code>zic</code>'s <code>%z</code> notation.
</li>
<li>
Use current abbreviations for older timestamps to avoid confusion.
For example, in 1910 a common English abbreviation for time
in central Europe was 'MEZ' (short for both "Middle European
Zone" and for "Mitteleuropäische Zeit" in German).
Nowadays 'CET' ("Central European Time") is more common in
English, and the database uses 'CET' even for circa-1910
timestamps as this is less confusing for modern users and avoids
the need for determining when 'CET' supplanted 'MEZ' in common
usage.
</li>
<li>
Use a consistent style in a timezone's history.
For example, if a history tends to use numeric
abbreviations and a particular entry could go either way, use a
numeric abbreviation.
</li>
<li>
Use
<a href="https://en.wikipedia.org/wiki/Universal_Time">Universal Time</a>
(<abbr>UT</abbr>) (with time zone abbreviation '<code>-</code>00') for
locations while uninhabited.
The leading '<code>-</code>' is a flag that the <abbr>UT</abbr> offset is in
some sense undefined; this notation is derived
from <a href="https://tools.ietf.org/html/rfc3339">Internet
<abbr title="Request For Comments">RFC</abbr> 3339</a>.
</li>
</ul>
<p>
Application writers should note that these abbreviations are ambiguous
in practice: e.g., 'CST' means one thing in China and something else
in North America, and 'IST' can refer to time in India, Ireland or
Israel.
To avoid ambiguity, use numeric <abbr>UT</abbr> offsets like
'<code>-</code>0600' instead of time zone abbreviations like 'CST'.
</p>
</section>
<section>
<h2 id="accuracy">Accuracy of the <code><abbr>tz</abbr></code> database</h2>
<p>
The <code><abbr>tz</abbr></code> database is not authoritative, and it
surely has errors.
Corrections are welcome and encouraged; see the file <code>CONTRIBUTING</code>.
Users requiring authoritative data should consult national standards
bodies and the references cited in the database's comments.
</p>
<p>
Errors in the <code><abbr>tz</abbr></code> database arise from many sources:
</p>
<ul>
<li>
The <code><abbr>tz</abbr></code> database predicts future
timestamps, and current predictions
will be incorrect after future governments change the rules.
For example, if today someone schedules a meeting for 13:00 next
October 1, Casablanca time, and tomorrow Morocco changes its
daylight saving rules, software can mess up after the rule change
if it blithely relies on conversions made before the change.
</li>
<li>
The pre-1970 entries in this database cover only a tiny sliver of how
clocks actually behaved; the vast majority of the necessary
information was lost or never recorded.
Thousands more timezones would be needed if
the <code><abbr>tz</abbr></code> database's scope were extended to
cover even just the known or guessed history of standard time; for
example, the current single entry for France would need to split
into dozens of entries, perhaps hundreds.
And in most of the world even this approach would be misleading
due to widespread disagreement or indifference about what times
should be observed.
In her 2015 book
<cite><a
href="http://www.hup.harvard.edu/catalog.php?isbn=9780674286146">The
Global Transformation of Time, 1870–1950</a></cite>,
Vanessa Ogle writes
"Outside of Europe and North America there was no system of time
zones at all, often not even a stable landscape of mean times,
prior to the middle decades of the twentieth century".
See: Timothy Shenk, <a
href="https://www.dissentmagazine.org/blog/booked-a-global-history-of-time-vanessa-ogle">Booked:
A Global History of Time</a>. <cite>Dissent</cite> 2015-12-17.
</li>
<li>
Most of the pre-1970 data entries come from unreliable sources, often
astrology books that lack citations and whose compilers evidently
invented entries when the true facts were unknown, without
reporting which entries were known and which were invented.
These books often contradict each other or give implausible entries,
and on the rare occasions when they are checked they are
typically found to be incorrect.
</li>
<li>
For the UK the <code><abbr>tz</abbr></code> database relies on
years of first-class work done by
Joseph Myers and others; see
"<a href="https://www.polyomino.org.uk/british-time/">History of
legal time in Britain</a>".
Other countries are not done nearly as well.
</li>
<li>
Sometimes, different people in the same city maintain clocks
that differ significantly.
Historically, railway time was used by railroad companies (which
did not always
agree with each other), church-clock time was used for birth
certificates, etc.
More recently, competing political groups might disagree about
clock settings. Often this is merely common practice, but
sometimes it is set by law.
For example, from 1891 to 1911 the <abbr>UT</abbr> offset in France
was legally <abbr>UT</abbr> +00:09:21 outside train stations and
<abbr>UT</abbr> +00:04:21 inside. Other examples include
Chillicothe in 1920, Palm Springs in 1946/7, and Jerusalem and
Ürümqi to this day.
</li>
<li>
Although a named location in the <code><abbr>tz</abbr></code>
database stands for the containing region, its pre-1970 data
entries are often accurate for only a small subset of that region.
For example, <code>Europe/London</code> stands for the United
Kingdom, but its pre-1847 times are valid only for locations that
have London's exact meridian, and its 1847 transition
to <abbr>GMT</abbr> is known to be valid only for the L&NW and
the Caledonian railways.
</li>
<li>
The <code><abbr>tz</abbr></code> database does not record the
earliest time for which a timezone's
data entries are thereafter valid for every location in the region.
For example, <code>Europe/London</code> is valid for all locations
in its region after <abbr>GMT</abbr> was made the standard time,
but the date of standardization (1880-08-02) is not in the
<code><abbr>tz</abbr></code> database, other than in commentary.
For many timezones the earliest time of
validity is unknown.
</li>
<li>
The <code><abbr>tz</abbr></code> database does not record a
region's boundaries, and in many cases the boundaries are not known.
For example, the timezone
<code>America/Kentucky/Louisville</code> represents a region
around the city of Louisville, the boundaries of which are
unclear.
</li>
<li>
Changes that are modeled as instantaneous transitions in the
<code><abbr>tz</abbr></code>
database were often spread out over hours, days, or even decades.
</li>
<li>
Even if the time is specified by law, locations sometimes
deliberately flout the law.
</li>
<li>
Early timekeeping practices, even assuming perfect clocks, were
often not specified to the accuracy that the
<code><abbr>tz</abbr></code> database requires.
</li>
<li>
The <code><abbr>tz</abbr></code> database cannot represent stopped clocks.
However, on 1911-03-11 at 00:00, some public-facing French clocks
were changed by stopping them for a few minutes to effect a transition.
The <code><abbr>tz</abbr></code> database models this via a
backward transition; the relevant French legislation does not
specify exactly how the transition was to occur.
</li>
<li>
Sometimes historical timekeeping was specified more precisely
than what the <code><abbr>tz</abbr></code> code can handle.
For example, from 1880 to 1916 clocks in Ireland observed Dublin Mean
Time (estimated to be <abbr>UT</abbr>
−00:25:21.1), but the <code><abbr>tz</abbr></code>
code cannot represent the fractional second.
In practice these old specifications were rarely if ever
implemented to subsecond precision.
</li>
<li>
Even when all the timestamp transitions recorded by the
<code><abbr>tz</abbr></code> database are correct, the
<code><abbr>tz</abbr></code> rules that generate them may not
faithfully reflect the historical rules.
For example, from 1922 until World War II the UK moved clocks
forward the day following the third Saturday in April unless that
was Easter, in which case it moved clocks forward the previous
Sunday.
Because the <code><abbr>tz</abbr></code> database has no
way to specify Easter, these exceptional years are entered as
separate <code><abbr>tz</abbr> Rule</code> lines, even though the
legal rules did not change.
When transitions are known but the historical rules behind them are not,
the database contains <code>Zone</code> and <code>Rule</code>
entries that are intended to represent only the generated
transitions, not any underlying historical rules; however, this
intent is recorded at best only in commentary.
</li>
<li>
The <code><abbr>tz</abbr></code> database models time
using the <a
href="https://en.wikipedia.org/wiki/Proleptic_Gregorian_calendar">proleptic
Gregorian calendar</a> with days containing 24 equal-length hours
numbered 00 through 23, except when clock transitions occur.
Pre-standard time is modeled as local mean time.
However, historically many people used other calendars and other timescales.
For example, the Roman Empire used
the <a href="https://en.wikipedia.org/wiki/Julian_calendar">Julian
calendar</a>,
and <a href="https://en.wikipedia.org/wiki/Roman_timekeeping">Roman
timekeeping</a> had twelve varying-length daytime hours with a
non-hour-based system at night.
And even today, some local practices diverge from the Gregorian
calendar with 24-hour days. These divergences range from
relatively minor, such as Japanese bars giving times like "24:30" for the
wee hours of the morning, to more-significant differences such as <a
href="https://www.pri.org/stories/2015-01-30/if-you-have-meeting-ethiopia-you-better-double-check-time">the
east African practice of starting the day at dawn</a>, renumbering
the Western 06:00 to be 12:00. These practices are largely outside
the scope of the <code><abbr>tz</abbr></code> code and data, which
provide only limited support for date and time localization
such as that required by POSIX.
If <abbr>DST</abbr> is not used a different time zone
can often do the trick; for example, in Kenya a <code>TZ</code> setting
like <code><-03>3</code> or <code>America/Cayenne</code> starts
the day six hours later than <code>Africa/Nairobi</code> does.
</li>
<li>
Early clocks were less reliable, and data entries do not represent
clock error.
</li>
<li>
The <code><abbr>tz</abbr></code> database assumes Universal Time
(<abbr>UT</abbr>) as an origin, even though <abbr>UT</abbr> is not
standardized for older timestamps.
In the <code><abbr>tz</abbr></code> database commentary,
<abbr>UT</abbr> denotes a family of time standards that includes
Coordinated Universal Time (<abbr>UTC</abbr>) along with other
variants such as <abbr>UT1</abbr> and <abbr>GMT</abbr>,
with days starting at midnight.
Although <abbr>UT</abbr> equals <abbr>UTC</abbr> for modern
timestamps, <abbr>UTC</abbr> was not defined until 1960, so
commentary uses the more-general abbreviation <abbr>UT</abbr> for
timestamps that might predate 1960.
Since <abbr>UT</abbr>, <abbr>UT1</abbr>, etc. disagree slightly,
and since pre-1972 <abbr>UTC</abbr> seconds varied in length,
interpretation of older timestamps can be problematic when
subsecond accuracy is needed.
</li>
<li>
Civil time was not based on atomic time before 1972, and we do not
know the history of
<a href="https://en.wikipedia.org/wiki/Earth's_rotation">earth's
rotation</a> accurately enough to map <a
href="https://en.wikipedia.org/wiki/International_System_of_Units"><abbr
title="International System of Units">SI</abbr></a> seconds to
historical <a href="https://en.wikipedia.org/wiki/Solar_time">solar time</a>
to more than about one-hour accuracy.
See: Stephenson FR, Morrison LV, Hohenkerk CY.
<a href="https://dx.doi.org/10.1098/rspa.2016.0404">Measurement of
the Earth's rotation: 720 BC to AD 2015</a>.
<cite>Proc Royal Soc A</cite>. 2016 Dec 7;472:20160404.
Also see: Espenak F. <a
href="https://eclipse.gsfc.nasa.gov/SEhelp/uncertainty2004.html">Uncertainty
in Delta T (ΔT)</a>.
</li>
<li>
The relationship between POSIX time (that is, <abbr>UTC</abbr> but
ignoring <a href="https://en.wikipedia.org/wiki/Leap_second">leap
seconds</a>) and <abbr>UTC</abbr> is not agreed upon after 1972.
Although the POSIX
clock officially stops during an inserted leap second, at least one
proposed standard has it jumping back a second instead; and in
practice POSIX clocks more typically either progress glacially during
a leap second, or are slightly slowed while near a leap second.
</li>
<li>
The <code><abbr>tz</abbr></code> database does not represent how
uncertain its information is.
Ideally it would contain information about when data entries are
incomplete or dicey.
Partial temporal knowledge is a field of active research, though,
and it is not clear how to apply it here.
</li>
</ul>
<p>
In short, many, perhaps most, of the <code><abbr>tz</abbr></code>
database's pre-1970 and future timestamps are either wrong or
misleading.
Any attempt to pass the
<code><abbr>tz</abbr></code> database off as the definition of time
should be unacceptable to anybody who cares about the facts.
In particular, the <code><abbr>tz</abbr></code> database's
<abbr>LMT</abbr> offsets should not be considered meaningful, and
should not prompt creation of timezones
merely because two locations
differ in <abbr>LMT</abbr> or transitioned to standard time at
different dates.
</p>
</section>
<section>
<h2 id="functions">Time and date functions</h2>
<p>
The <code><abbr>tz</abbr></code> code contains time and date functions
that are upwards compatible with those of POSIX.
Code compatible with this package is already
<a href="tz-link.html#tzdb">part of many platforms</a>, where the
primary use of this package is to update obsolete time-related files.
To do this, you may need to compile the time zone compiler
'<code>zic</code>' supplied with this package instead of using the
system '<code>zic</code>', since the format of <code>zic</code>'s
input is occasionally extended, and a platform may still be shipping
an older <code>zic</code>.
</p>
<h3 id="POSIX">POSIX properties and limitations</h3>
<ul>
<li>
<p>
In POSIX, time display in a process is controlled by the
environment variable <code>TZ</code>.
Unfortunately, the POSIX
<code>TZ</code> string takes a form that is hard to describe and
is error-prone in practice.
Also, POSIX <code>TZ</code> strings cannot deal with daylight
saving time rules not based on the Gregorian calendar (as in
Iran), or with situations where more than two time zone
abbreviations or <abbr>UT</abbr> offsets are used in an area.
</p>
<p>
The POSIX <code>TZ</code> string takes the following form:
</p>
<p>
<var>stdoffset</var>[<var>dst</var>[<var>offset</var>][<code>,</code><var>date</var>[<code>/</code><var>time</var>]<code>,</code><var>date</var>[<code>/</code><var>time</var>]]]
</p>
<p>
where:
</p>
<dl>
<dt><var>std</var> and <var>dst</var></dt><dd>
are 3 or more characters specifying the standard
and daylight saving time (<abbr>DST</abbr>) zone abbreviations.
Starting with POSIX.1-2001, <var>std</var> and <var>dst</var>
may also be in a quoted form like '<code><+09></code>';
this allows "<code>+</code>" and "<code>-</code>" in the names.
</dd>
<dt><var>offset</var></dt><dd>
is of the form
'<code>[±]<var>hh</var>:[<var>mm</var>[:<var>ss</var>]]</code>'
and specifies the offset west of <abbr>UT</abbr>.
'<var>hh</var>' may be a single digit;
0≤<var>hh</var>≤24.
The default <abbr>DST</abbr> offset is one hour ahead of
standard time.
</dd>
<dt><var>date</var>[<code>/</code><var>time</var>]<code>,</code><var>date</var>[<code>/</code><var>time</var>]</dt><dd>
specifies the beginning and end of <abbr>DST</abbr>.
If this is absent, the system supplies its own ruleset
for <abbr>DST</abbr>, and its rules can differ from year to year;
typically <abbr>US</abbr> <abbr>DST</abbr> rules are used.
</dd>
<dt><var>time</var></dt><dd>
takes the form
'<var>hh</var><code>:</code>[<var>mm</var>[<code>:</code><var>ss</var>]]'
and defaults to 02:00.
This is the same format as the offset, except that a
leading '<code>+</code>' or '<code>-</code>' is not allowed.
</dd>
<dt><var>date</var></dt><dd>
takes one of the following forms:
<dl>
<dt>J<var>n</var> (1≤<var>n</var>≤365)</dt><dd>
origin-1 day number not counting February 29
</dd>
<dt><var>n</var> (0≤<var>n</var>≤365)</dt><dd>
origin-0 day number counting February 29 if present
</dd>
<dt><code>M</code><var>m</var><code>.</code><var>n</var><code>.</code><var>d</var>
(0[Sunday]≤<var>d</var>≤6[Saturday], 1≤<var>n</var>≤5,
1≤<var>m</var>≤12)</dt><dd>
for the <var>d</var>th day of week <var>n</var> of
month <var>m</var> of the year, where week 1 is the first
week in which day <var>d</var> appears, and
'<code>5</code>' stands for the last week in which
day <var>d</var> appears (which may be either the 4th or
5th week).
Typically, this is the only useful form; the <var>n</var>
and <code>J</code><var>n</var> forms are rarely used.
</dd>
</dl>
</dd>
</dl>
<p>
Here is an example POSIX <code>TZ</code> string for New
Zealand after 2007.
It says that standard time (<abbr>NZST</abbr>) is 12 hours ahead
of <abbr>UT</abbr>, and that daylight saving time
(<abbr>NZDT</abbr>) is observed from September's last Sunday at
02:00 until April's first Sunday at 03:00:
</p>
<pre><code>TZ='NZST-12NZDT,M9.5.0,M4.1.0/3'</code></pre>
<p>
This POSIX <code>TZ</code> string is hard to remember, and
mishandles some timestamps before 2008.
With this package you can use this instead:
</p>
<pre><code>TZ='Pacific/Auckland'</code></pre>
</li>
<li>
POSIX does not define the <abbr>DST</abbr> transitions
for <code>TZ</code> values like
"<code>EST5EDT</code>".
Traditionally the current <abbr>US</abbr> <abbr>DST</abbr> rules
were used to interpret such values, but this meant that the
<abbr>US</abbr> <abbr>DST</abbr> rules were compiled into each
program that did time conversion. This meant that when
<abbr>US</abbr> time conversion rules changed (as in the United
States in 1987), all programs that did time conversion had to be
recompiled to ensure proper results.
</li>
<li>
The <code>TZ</code> environment variable is process-global, which
makes it hard to write efficient, thread-safe applications that
need access to multiple timezones.
</li>
<li>
In POSIX, there is no tamper-proof way for a process to learn the
system's best idea of local (wall clock) time.
This is important for applications that an administrator wants
used only at certain times – without regard to whether the
user has fiddled the
<code>TZ</code> environment variable.
While an administrator can "do everything in <abbr>UT</abbr>" to
get around the problem, doing so is inconvenient and precludes
handling daylight saving time shifts – as might be required to
limit phone calls to off-peak hours.
</li>
<li>
POSIX provides no convenient and efficient way to determine
the <abbr>UT</abbr> offset and time zone abbreviation of arbitrary
timestamps, particularly for timezones
that do not fit into the POSIX model.
</li>
<li>
POSIX requires that <code>time_t</code> clock counts exclude leap
seconds.
</li>
<li>
The <code><abbr>tz</abbr></code> code attempts to support all the
<code>time_t</code> implementations allowed by POSIX.
The <code>time_t</code> type represents a nonnegative count of seconds
since 1970-01-01 00:00:00 <abbr>UTC</abbr>, ignoring leap seconds.
In practice, <code>time_t</code> is usually a signed 64- or 32-bit
integer; 32-bit signed <code>time_t</code> values stop working after
2038-01-19 03:14:07 <abbr>UTC</abbr>, so new implementations these