[tz] draft of change summary for next tz release

[Paul Eggert]

On 09/12/13 08:39, Tim Parenti wrote:

> it would be quite prudent to summarize specifically what changes
> are (and are not) made by this changeset

Yes, although I normally write a change summary when preparing a
release and publish it as part of the release announcement, the
proposed change is large enough that it'd be helpful to circulate
a draft of the change summary now.  Here is a draft for the
current set of proposed changes, which is:

https://github.com/eggert/tz/compare/8f10e5c...738ad89

  Changes affecting near-future time stamps

    This year Fiji will start DST on October 27, not October 20.
    (Thanks to David Wheeler for the heads-up.)  For now, guess that
    Fiji will continue to spring forward the Sunday before the fourth
    Monday in October.

  Changes affecting current and future time zone abbreviations

    Use WIB/WITA/WIT rather than WIT/CIT/EIT for alphabetic Indonesian
    time zone abbreviations since 1932.  (Thanks to George Ziegler,
    Priyadi Iman Nurcahyo, Zakaria, Jason Grimes, Martin Pitt, and
    Benny Lin.)  This affects Asia/Dili, Asia/Jakarta, Asia/Jayapura,
    Asia/Makassar, and Asia/Pontianak.

    Use ART (UTC-3, standard time), rather than WARST (also UTC-3, but
    daylight saving time) for San Luis, Argentina since 2009.

  Changes affecting Godthab time stamps after 2037 if version mismatch

    Allow POSIX-like TZ strings where the transition time's hour can
    range from -167 through 167, instead of the POSIX-required 0
    through 24.  E.g., TZ='FJT-12FJST,M10.3.1/146,M1.3.4/75' for the
    new Fiji rules.  This is a more-compact way to represent
    far-future time stamps for America/Godthab, America/Santiago,
    Antarctica/Palmer, Asia/Gaza, Asia/Hebron, Asia/Jerusalem,
    Pacific/Easter, and Pacific/Fiji.  Other zones are unaffected by
    this change.  (Derived from a suggestion by Arthur David Olson.)

    Allow POSIX-like TZ strings where daylight saving time is in
    effect all year.  E.g., TZ='WART4WARST,J1/0,J365/25' for Western
    Argentina Summer Time all year.  This supports a more-compact way
    to represent the 2013d data for America/Argentina/San_Luis.
    Because of the change for San Luis noted above this change does not
    affect the current data.  (Thanks to Andrew Main (Zefram) for
    suggestions that improved this change.)

    Where these two TZ changes take effect, there is a minor extension
    to the tz file format in that it allows new values for the
    embedded TZ-format string, and the tz file format version number
    has therefore been increased from 2 to 3 as a precaution.
    Version-2-based client code should continue to work as before for
    all time stamps before 2038.  Existing version-2-based client code
    (tzcode, GNU/Linux, Solaris) has been tested on version-3-format
    files, and typically works in practice even for time stamps after
    2037; the only known exception is America/Godthab.

  Changes affecting time stamps before 1970

    Pacific/Johnston is now a link to Pacific/Honolulu.  This corrects
    some errors before 1947.

    Some zones have been turned into links, when they differ from
    existing zones only in older data that was likely invented or that
    differs only in LMT or transition from LMT.  These changes affect
    only time stamps before 1943.  The affected zones are:
    Africa/Juba, America/Anguilla, America/Aruba, America/Dominica,
    America/Grenada, America/Guadeloupe, America/Marigot,
    America/Montserrat, America/St_Barthelemy, America/St_Kitts,
    America/St_Lucia, America/St_Thomas, America/St_Vincent,
    America/Tortola, and Europe/Vaduz.

    Change Kingston Mean Time from -5:07:12 to -5:07:11.  This affects
    America/Cayman, America/Jamaica and America/Grand_Turk time stamps
    from 1890 to 1912.

    Change the UT offset of Bern Mean Time from 0:29:44 to 0:29:46.
    This affects Europe/Zurich time stamps from 1853 to 1894.  (Thanks
    to Alois Treindl).

    Change the date of the circa-1850 Zurich transition from 1849-09-12
    to 1853-07-16, overriding Shanks with data from Messerli about
    postal and telegraph time in Switzerland.

  Changes affecting time zone abbreviations before 1970

    For Asia/Jakarta, use BMT (not JMT) for mean time from 1923 to 1932,
    as Jakarta was called Batavia back then.

  Changes affecting API

    The 'zic' command now outputs a dummy transition when far-future
    data can't be summarized using a TZ string, and uses a 402-year
    window rather than a 400-year window.  For the current data, this
    affects only the Asia/Tehran file.  It does not affect any of the
    time stamps that this file represents, so zdump outputs the same
    information as before.  (Thanks to Andrew Main (Zefram).)

    The 'date' command has a new '-r' option, which lets you specify
    the integer time to display, a la FreeBSD.

    The 'tzselect' command has two new options '-c' and '-n', which lets you
    select a zone based on latitude and longitude.

    The 'zic' command's '-v' option now warns about constructs that
    require the new version-3 binary file format.  (Thanks to Arthur
    David Olson for the suggestion.)

    Support for floating-point time_t has been removed.
    It was always dicey, and POSIX no longer requires it.
    (Thanks to Eric Blake for suggesting to the POSIX committee to
    remove it, and thanks to Alan Barrett, Clive D.W. Feather, Andy
    Heninger, Arthur David Olson, and Alois Treindl, for reporting
    bugs and elucidating some of the corners of the old floating-point
    implementation.)

    The signatures of 'offtime', 'timeoff', and 'gtime' have been
    changed back to the old practice of using 'long' to represent UT
    offsets.  This had been inadvertently and mistakenly changed to
    'int_fast32_t'.  (Thanks to Christos Zoulos.)

    The code avoids undefined behavior on integer overflow in some
    more places, including gmtime, localtime, mktime and zdump.

  Changes affecting the zdump utility

    zdump now outputs "UT" when referring to Universal Time, not "UTC".
    "UTC" does not make sense for time stamps that predate the introduction
    of UTC, whereas "UT", a more-generic term, does.  (Thanks to Steve Allen
    for clarifying UT vs UTC.)

  Data changes affecting behavior of tzselect and similar programs

    Country code BQ is now called the more-common name "Caribbean
Netherlands"
    rather than the more-official "Bonaire, St Eustatius & Saba".

    Remove from zone.tab the names America/Montreal, America/Shiprock,
    and Antarctica/South_Pole, as they are equivalent to existing
    same-country-code zones for post-1970 time stamps.  The data for
    these names are unchanged, so the names continue to work as before.

  Changes affecting code internals

    zic -c now runs way faster on 64-bit hosts when given large numbers.

    zic now uses vfprintf to allocating and freeing some memory.

    tzselect now computes the list of continents from the data,
    rather than have it hard-coded.

    Minor changes pacify GCC 4.7.3 and GCC 4.8.1.

  Changes affecting the build procedure

    The 'leapseconds' file is now generated automatically from a
    new file 'leap-seconds.list', which is a copy of
    <ftp://time.nist.gov/pub/leap-seconds.list>.
    A new source file 'leapseconds.awk' implements this.
    The goal is simplification of the future maintenance of 'leapseconds'.

    When building the 'posix' or 'right' subdirectories, if the
    subdirectory would be a copy of the default subdirectory, it is
    now made a symbolic link if that is supported.  This saves about
    2 MB of file system space.

    The links America/Shiprock and Antarctica/South_Pole have been
    moved to the 'backward' file.  This affects only nondefault builds
    that omit 'backward'.

  Changes affecting version-control only

    .gitignore now ignores 'date'.

  Changes affecting documentation and commentary

    Changes to the 'tzfile' man page

      It now mentions that the binary file format may be extended in
      future versions by appending data.

      It now refers to the 'zdump' and 'zic' man pages.

    Changes to the 'zic' man page

      It lists conditions that elicit a warning with '-v'.

      It says that the behavior is unspecified when duplicate names
      are given, or if the source of one link is the target of another.

      Its examples are updated to match the latest data.

      The definition of white space has been clarified slightly.
      (Thanks to Michael Deckers.)

    Changes to the 'Theory' file

      There is a new section about the accuracy of the tz database,
      describing the many ways that errors can creep in, and
      explaining why so many of the pre-1970 time stamps are wrong or
      misleading (thanks to Steve Allen, Lester Caine, and Garrett
      Wollman for discussions that contributed to this).

      The 'Theory' file describes LMT better (this follows a
      suggestion by Guy Harris).

      It refers to the 2013 edition of POSIX rather than the 2004 edition.

      It's mentioned that excluding 'backward' should not affect the
      other data, and it suggests at least one zone.tab name per
      inhabited country (thanks to Stephen Colebourne).

      Some longstanding restrictions on names are documented, e.g.,
      'America/New_York' precludes 'America/New_York/Bronx'.

      It gives more reasons for the 1970 cutoff.

      It now mentions which time_t variants are supported, such as
      signed integer time_t.  (Thanks to Paul Goyette for reporting
      typos in an experimental version of this change.)

      (Thanks to Philip Newton for correcting typos in these changes.)

    Documentation and commentary is more careful to distinguish UT in
    general from UTC in particular.  (Thanks to Steve Allen.)

    Add a better source for the Zurich 1894 transition.
    (Thanks to Pierre-Yves Berger.)

    Update shapefile citations in tz-link.htm.  (Thanks to Guy Harris.)