[Fri Mar 23 22:09:23 UTC 2018]

The 2018d release of the tz code and data is available. It reflects the 
following changes, which were either circulated on the tz mailing list 
or are relatively minor technical or administrative changes:

   Briefly:

   Palestine starts DST a week earlier in 2018.
   Add support for vanguard and rearguard data consumers.
   Add subsecond precision to source data format, though not to data.

   Changes to future time stamps

     In 2018, Palestine starts DST on March 24, not March 31.
     Adjust future predictions accordingly.  (Thanks to Sharef Mustafa.)

   Changes to past and future time stamps

     Casey Station in Antarctica changed from +11 to +08 on 2018-03-11
     at 04:00.  (Thanks to Steffen Thorsen.)

   Changes to past time stamps

     Historical transitions for Uruguay, represented by
     America/Montevideo, have been updated per official legal documents,
     replacing previous data mainly originating from the inventions of
     Shanks & Pottenger.  This has resulted in adjustments ranging from
     30 to 90 minutes in either direction over at least two dozen
     distinct periods ranging from one day to several years in length.
     A mere handful of pre-1991 transitions are unaffected; data since
     then has come from more reliable contemporaneous reporting. These
     changes affect various timestamps in 1920-1923, 1936, 1939,
     1942-1943, 1959, 1966-1970, 1972, 1974-1980, and 1988-1990.
     Additionally, Uruguay's pre-standard-time UT offset has been
     adjusted westward by 7 seconds, from UT-03:44:44 to UT-03:44:51, to
     match the location of the Observatory of the National Meteorological
     Institute in Montevideo.
     (Thanks to Jeremie Bonjour, Tim Parenti, and Michael Deckers.)

     Enderbury and Kiritimati skipped New Year's Eve 1994, not
     New Year's Day 1995.  (Thanks to Kerry Shetline.)

     Fix the 1912-01-01 transition for Portugual and its colonies.
     This transition was at 00:00 according to the new UT offset, not
     according to the old one.  Also assume that Cape Verde switched on
     the same date as the rest, not in 1907.  This affects
     Africa/Bissau, Africa/Sao_Tome, Asia/Macau, Atlantic/Azores,
     Atlantic/Cape_Verde, Atlantic/Madeira, and Europe/Lisbon.
     (Thanks to Michael Deckers.)

     Fix an off-by-1 error for pre-1913 timestamps in Jamaica and in
     Turks & Caicos.

   Changes to past time zone abbreviations

     MMT took effect in Uruguay from 1908-06-10, not 1898-06-28. There
     is no clock change associated with the transition.

   Changes to build procedure

     The new DATAFORM macro in the Makefile lets the installer choose
     among three source data formats.  The idea is to lessen downstream
     disruption when data formats are improved.

     * DATAFORM=vanguard installs from the latest, bleeding-edge
       format.  DATAFORM=main (the default) installs from the format
       used in the 'africa' etc. files.  DATAFORM=rearguard installs
       from a trailing-edge format.  Eventually, elements of today's
       vanguard format should move to the main format, and similarly
       the main format's features should eventually move to the
       rearguard format.

     * In the current version, the main and rearguard formats are
       identical and match that of 2018c, so this change does not
       affect default behavior.  The vanguard format currently contains
       one feature not in the main format: negative SAVE values. This
       improves support for Ireland, which uses Irish Standard Time
       (IST, UTC+01) in summer and GMT (UTC) in winter.  tzcode has
       supported negative SAVE values for decades, and this feature
       should move to the main format soon.  However, it will not move
       to the rearguard format for quite some time because some
       downstream parsers do not support it.

     * The build procedure constructs three files vanguard.zi, main.zi,
       and rearguard.zi, one for each format.  The files represent the
       same data as closely as the formats allow.  These three files
       are intended for downstream data consumers and are not
       installed.  Zoneinfo parsers that do not support negative SAVE values
       should start using rearguard.zi, so that they will be unaffected
       when the negative-DST feature moves from vanguard to main.
       Bleeding-edge Zoneinfo parsers that support the new features
       already can use vanguard.zi; in this respect, current tzcode is
       bleeding-edge.

     The Makefile should now be safe for parallelized builds, and 'make
     -j to2050new.tzs' is now much faster on a multiprocessor host
     with GNU Make.

     When built with -DSUPPRESS_TZDIR, the tzcode library no longer
     prepends TZDIR/ to file names that do not begin with '/'. This is
     not recommended for general use, due to its security implications.
     (From a suggestion by Manuela Friedrich.)

   Changes to code

     zic now accepts subsecond precision in expressions like
     00:19:32.13, which is approximately the legal time of the
     Netherlands from 1835 to 1937.  However, because it is
     questionable whether the few recorded uses of non-integer offsets
     had subsecond precision in practice, there are no plans for tzdata
     to use this feature.  (Thanks to Steve Allen for pointing out
     the limitations of historical data in this area.)

     The code is a bit more portable to MS-Windows.  Installers can
     compile with -DRESERVE_STD_EXT_IDS on MS-Windows platforms that
     reserve identifiers like 'localtime'.  (Thanks to Manuela
     Friedrich).

   Changes to documentation and commentary

     theory.html now outlines tzdb's extensions to POSIX's model for
     civil time, and has a section "POSIX features no longer needed"
     that lists POSIX API components that are now vestigial.
     (From suggestions by Steve Summit.)  It also better distinguishes
     time zones from tz regions.  (From a suggestion by Guy Harris.)

     Commentary is now more consistent about using the phrase "daylight
     saving time", to match the C name tm_isdst.  Daylight saving time
     need not occur in summer, and need not have a positive offset from
     standard time.

     Commentary about historical transitions in Uruguay has been expanded
     with links to many relevant legal documents.
     (Thanks to Tim Parenti.)

     Commentary now uses some non-ASCII characters with Unicode value
     less than U+0100, as they can be useful and should work even with
     older editors such as XEmacs.

Here are links to the release files:

https://www.iana.org/time-zones/repository/releases/tzcode2018d.tar.gz
https://www.iana.org/time-zones/repository/releases/tzdata2018d.tar.gz
https://www.iana.org/time-zones/repository/releases/tzdb-2018d.tar.lz

Links are also available via plain HTTP, and via FTP from
ftp://ftp.iana.org/tz/releases with the same basenames as above.