xref: /src/lib/libc/time/README

@(#)README	8.3
This file is in the public domain, so clarified as of
2009-05-17 by Arthur David Olson.

"What time is it?" -- Richard Deacon as The King
"Any time you want it to be." -- Frank Baxter as The Scientist
					(from the Bell System film "About Time")

The 1989 update of the time zone package featured

*	POSIXization (including interpretation of POSIX-style TZ environment
	variables, provided by Guy Harris),
*	ANSIfication (including versions of "mktime" and "difftime"),
*	SVIDulation (an "altzone" variable)
*	MACHination (the "gtime" function)
*	corrections to some time zone data (including corrections to the rules
	for Great Britain and New Zealand)
*	reference data from the United States Naval Observatory for folks who
	want to do additional time zones
*	and the 1989 data for Saudi Arabia.

(Since this code will be treated as "part of the implementation" in some places
and as "part of the application" in others, there's no good way to name
functions, such as timegm, that are not part of the proposed ANSI C standard;
such functions have kept their old, underscore-free names in this update.)

And the "dysize" function has disappeared; it was present to allow compilation
of the "date" command on old BSD systems, and a version of "date" is now
provided in the package.  The "date" command is not created when you "make all"
since it may lack options provided by the version distributed with your
operating system, or may not interact with the system in the same way the
native version does.

Since POSIX frowns on correct leap second handling, the default behavior of
the "zic" command (in the absence of a "-L" option) has been changed to omit
leap second information from its output files.

Here is a recipe for acquiring, building, installing, and testing the
tz distribution on a GNU/Linux or similar host.

	mkdir tz
	cd tz
	wget 'ftp://elsie.nci.nih.gov/pub/tz*.tar.gz'
	gzip -dc tzcode*.tar.gz | tar -xf -
	gzip -dc tzdata*.tar.gz | tar -xf -

Be sure to read the comments in "Makefile" and make any changes needed
to make things right for your system, especially if you are using some
platform other than GNU/Linux.  Then run the following commands,
substituting your desired installation directory for "$HOME/tzdir":

	make TOPDIR=$HOME/tzdir install
	$HOME/tzdir/etc/zdump -v America/Los_Angeles

To use the new functions, use a "-ltz" option when compiling or linking.

Historical local time information has been included here to:

*	provide a compendium of data about the history of civil time
	that is useful even if the data are not 100% accurate;

*	give an idea of the variety of local time rules that have
	existed in the past and thus an idea of the variety that may be
	expected in the future;

*	provide a test of the generality of the local time rule description
	system.

The information in the time zone data files is by no means authoritative;
the files currently do not even attempt to cover all time stamps before
1970, and there are undoubtedly errors even for time stamps since 1970.
If you know that the rules are different from those in a file, by all means
feel free to change file (and please send the changed version to
tz@elsie.nci.nih.gov for use in the future).  Europeans take note!

Thanks to these Timezone Caballeros who've made major contributions to the
time conversion package:  Keith Bostic; Bob Devine; Paul Eggert; Robert Elz;
Guy Harris; Mark Horton; John Mackin; and Bradley White.  Thanks also to
Michael Bloom, Art Neilson, Stephen Prince, John Sovereign, and Frank Wales
for testing work, and to Gwillim Law for checking local mean time data.
None of them are responsible for remaining errors.

Look in the ~ftp/pub directory of elsie.nci.nih.gov
for updated versions of these files.

Please send comments or information to tz@elsie.nci.nih.gov.