README for tzdata / tzdata-legacy
=================================

The tzdata and tzdata-legacy packages contain data required for the
implementation of standard local time for many representative locations around
the globe. Timezones are typically identified by continent or ocean, followed
by the name of the largest city within the region. This approach helps avoid
political issues that could arise from selecting timezones based on country,
region, or timezone abbreviation.


tzdata package
~~~~~~~~~~~~~~
The tzdata package contains the "POSIX" version of the timezone data that most
users will use, as their system hardware clock is set either to UTC or to a
timezone with a fixed offset from Coordinated Universal Time (UTC). For this
reason, it does not include leap seconds, and their possible correction must be
made by another system (e.g., NTP).

The Debconf interface (see section below) allows users to select from all the
timezones available in the tzdata package.


tzdata-legacy package
~~~~~~~~~~~~~~~~~~~~~
The tzdata-legacy package contains:

- Additional timezones that have been deprecated upstream. These may include
  timezones that are not following the current rule of using the geographical
  region (continent or ocean) and city name, a city whose name has changed, or
  a deprecated timezone abbreviation. These zones are not selectable using the
  Debconf interface but may be useful for certain software or testsuites. They
  can also be used by setting the TZ environment variable to the name of the
  timezone.

- The "right" version of both the legacy and non-legacy timezones. This version
  accounts for leap seconds, diverging from the "posix" version starting in
  June 1972, when the first leap second was introduced. It has a fixed -10
  second offset from International Atomic Time (TAI) and, as of 2025, a
  27-second difference from UTC. This version is suitable for systems with a
  hardware clock set to TAI-10s, which is the current IBM recommended setting
  on IBM mainframes. To use it, set the TZDIR environment variable to
  /usr/share/zoneinfo/right.


Configuring the system's timezone interactively
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The timezone configuration can be done interactively using Debconf by
reconfiguring the tzdata package, with the following command:

  dpkg-reconfigure tzdata


Configuring the system's timezone programmatically
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
To programmatically read or configure the system's timezone, it is recommended
to use the systemd interface:
- To show the current timezone:
  timedatectl show --value --property=Timezone
- To set the timezone:
  timedatectl set-timezone Europe/Paris

Alternatively, you can use the following commands to read and update the
/etc/localtime symbolic link:
- To read the current timezone:
  readlink -f /etc/localtime  | sed -e 's,/usr/share/zoneinfo/,,'
- To set the timezone:
  ln -nsf /usr/share/zoneinfo/Europe/Paris /etc/localtime