NetBSD/lib/libc/time/Theory

#	$NetBSD: Theory,v 1.3 1998/01/22 07:06:57 jtc Exp $
@(#)Theory	7.6


----- 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.

	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 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 UTC" 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.)

*	POSIX.1 requires that systems ignore leap seconds.

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
	POSIX); "TZ" is no longer constrained to be a three-letter time zone
	name followed by a number of hours and an optional three-letter
	daylight time zone name.  The daylight saving time rules to be used
	for a particular time zone are encoded in the time zone file;
	the format of the file allows U.S., Australian, and other rules to be
	encoded, and allows for situations where more than two time zone
	abbreviations are used.

	It was recognized that allowing the "TZ" environment variable to
	take on values such as "America/New_York" might cause "old" programs
	(that expect "TZ" to have a certain form) to operate incorrectly;
	consideration was given to using some other environment variable
	(for example, "TIMEZONE") to hold the string used to generate the
	time zone information file name.  In the end, however, it was decided
	to continue using "TZ":  it is widely used for time zone purposes;
	separately maintaining both "TZ" and "TIMEZONE" seemed a nuisance;
	and systems where "new" forms of "TZ" might cause problems can simply
	use TZ values such as "EST5EDT" which can be used both by
	"new" programs (a la POSIX) and "old" programs (as zone names and
	offsets).

*	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 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.  (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
	for near-minimum or near-maximum time_t values.  (A comment in the
	source code tells how to get compatibly wrong results).

*	A function "tzsetwall" has been added to arrange for the system's
	best approximation to local wall clock time to be delivered by
	subsequent calls to "localtime."  Source code for portable
	applications that "must" run on local wall clock time should call
	"tzsetwall();" if such code is moved to "old" systems that don't
	provide tzsetwall, you won't be able to generate an executable program.
	(These time zone functions also arrange for local wall clock time to be
	used if tzset is called--directly or indirectly--and there's no "TZ"
	environment variable; portable applications should not, however, rely
	on this behavior since it's not the way SVR2 systems behave.)

*	These functions can account for leap seconds, thanks to Bradley White
	(bww@k.cs.cmu.edu).

Points of interest to folks with other systems:

*	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.
	Programs that in the past used the timezone function may now examine
	tzname[localtime(&clock)->tm_isdst] to learn the correct time
	zone abbreviation to use.  Alternatively, use
	localtime(&clock)->tm_zone if this has been enabled.

*	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 UTC.
	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
not in any sense "standard compatible"--some are not, in fact, specified in
*any* standard.  They do, however, represent responses of various authors to
standardization proposals.

Other time conversion proposals, in particular the one developed by folks at
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
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 UTC offsets like `-0500' instead of time zone
abbreviations like `EST'; this avoids the ambiguity.