[Bug 2003797] Re: Pre-1970 timestamps are knowingly wrong

[Benjamin Drung]

** Description changed:

  tzdata 2021b says in NEWS: "Merge more location-based Zones whose
  timestamps agree since 1970, as pre-1970 timestamps are out of scope.
  This is part of a process that has been ongoing since 2013.  This does
  not affect post-1970 timestamps, and timezone historians who build with
  'make PACKRATDATA=backzone' should see no changes to pre-1970
  timestamps." tzdata 2022b finished that process: "Finish moving to
  'backzone' the location-based zones whose timestamps since 1970 are
  duplicates; adjust links accordingly. This change ordinarily affects
  only pre-1970 timestamps, and with the new PACKRATLIST option it does
  not affect any timestamps. In this round the affected zones are
  Antarctica/Vostok, Asia/Brunei, Asia/Kuala_Lumpur, Atlantic/Reykjavik,
  Europe/Amsterdam, Europe/Copenhagen, Europe/Luxembourg, Europe/Monaco,
  Europe/Oslo, Europe/Stockholm, Indian/Christmas, Indian/Cocos,
  Indian/Kerguelen, Indian/Mahe, Indian/Reunion, Pacific/Chuuk,
  Pacific/Funafuti, Pacific/Majuro, Pacific/Pohnpei, Pacific/Wake and
  Pacific/Wallis, and the affected links are Arctic/Longyearbyen,
  Atlantic/Jan_Mayen, Iceland, Pacific/Ponape, Pacific/Truk, and
  Pacific/Yap."
  
  The Debian/Ubuntu package builds tzdata with the default settings
  (PACKRATDATA unset) which merges zones whose timestamps agree since
  1970. This leads to timestamps before 1970 being knowingly wrong.
  Example: Europe/Oslo is a link to Europe/Berlin since they are identical
  since 1970, but they differed before 1965. Berlin observed no summertime
  between 1950 and 1980, but Oslo did in 1959 to 1965. So summer 1960 in
  Oslo should be UTC+2 compared to UTC+1 in Berlin:
  
  ```
  $ TZ=Europe/Oslo date -d "1960-07-01 10:00 Z"
  Fri Jul  1 12:00:00 CEST 1960
  $ TZ=Europe/Berlin date -d "1960-07-01 10:00 Z"
  Fri Jul  1 11:00:00 CET 1960
  ```
  
  There was a big debate upstream (see https://lwn.net/Articles/870478/ or
  https://mm.icann.org/pipermail/tz/2021-May/030137.html). The
  Debian/Ubuntu package should ship timezones from backzone. Users that
  care about pre-1970 timestamps will be satisfied and users not caring
  will probably not care about the additional timezones.
  
- The install size will increase by around 20% and the amount of
- distincted (i.e. not symlinked) timezones will increase by 33%:
+ The install size will increase by around 20% and the amount of distinct
+ (i.e. not symlinked) timezones will increase by 33%:
  
  ```
  $ grep ^Zone africa antarctica asia australasia etcetera europe factory northamerica southamerica | wc -l
  352
  $ grep ^Zone backzone | wc -l
  119
  ```

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https://bugs.launchpad.net/bugs/2003797

Title:
  Pre-1970 timestamps are knowingly wrong

Status in tzdata package in Ubuntu:
  Triaged

Bug description:
  tzdata 2021b says in NEWS: "Merge more location-based Zones whose
  timestamps agree since 1970, as pre-1970 timestamps are out of scope.
  This is part of a process that has been ongoing since 2013.  This does
  not affect post-1970 timestamps, and timezone historians who build
  with 'make PACKRATDATA=backzone' should see no changes to pre-1970
  timestamps." tzdata 2022b finished that process: "Finish moving to
  'backzone' the location-based zones whose timestamps since 1970 are
  duplicates; adjust links accordingly. This change ordinarily affects
  only pre-1970 timestamps, and with the new PACKRATLIST option it does
  not affect any timestamps. In this round the affected zones are
  Antarctica/Vostok, Asia/Brunei, Asia/Kuala_Lumpur, Atlantic/Reykjavik,
  Europe/Amsterdam, Europe/Copenhagen, Europe/Luxembourg, Europe/Monaco,
  Europe/Oslo, Europe/Stockholm, Indian/Christmas, Indian/Cocos,
  Indian/Kerguelen, Indian/Mahe, Indian/Reunion, Pacific/Chuuk,
  Pacific/Funafuti, Pacific/Majuro, Pacific/Pohnpei, Pacific/Wake and
  Pacific/Wallis, and the affected links are Arctic/Longyearbyen,
  Atlantic/Jan_Mayen, Iceland, Pacific/Ponape, Pacific/Truk, and
  Pacific/Yap."

  The Debian/Ubuntu package builds tzdata with the default settings
  (PACKRATDATA unset) which merges zones whose timestamps agree since
  1970. This leads to timestamps before 1970 being knowingly wrong.
  Example: Europe/Oslo is a link to Europe/Berlin since they are
  identical since 1970, but they differed before 1965. Berlin observed
  no summertime between 1950 and 1980, but Oslo did in 1959 to 1965. So
  summer 1960 in Oslo should be UTC+2 compared to UTC+1 in Berlin:

  ```
  $ TZ=Europe/Oslo date -d "1960-07-01 10:00 Z"
  Fri Jul  1 12:00:00 CEST 1960
  $ TZ=Europe/Berlin date -d "1960-07-01 10:00 Z"
  Fri Jul  1 11:00:00 CET 1960
  ```

  There was a big debate upstream (see https://lwn.net/Articles/870478/
  or https://mm.icann.org/pipermail/tz/2021-May/030137.html). The
  Debian/Ubuntu package should ship timezones from backzone. Users that
  care about pre-1970 timestamps will be satisfied and users not caring
  will probably not care about the additional timezones.

  The install size will increase by around 20% and the amount of
  distinct (i.e. not symlinked) timezones will increase by 33%:

  ```
  $ grep ^Zone africa antarctica asia australasia etcetera europe factory northamerica southamerica | wc -l
  352
  $ grep ^Zone backzone | wc -l
  119
  ```

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