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Local time and time zones for Nerves devices

NervesTimeZones provides a way of managing local time on embedded devices. It provides the following:

  1. Set your time zone and have it be used for local time calls like NaiveDateTime.local_now/0. The time zone persists across reboots.
  2. Set up Elixir's Calendar time zone database using zoneinfo
  3. Provide a small time zone database appropriate for many embedded devices

It does not support the automatic update of the time zone database like tzdata and tz. For now, you'll need to watch for new versions of the nerves_time_zones package. (We're open to changing this, but it's not as easy as regularly polling IANA.)

The primary motivation for creating this library was to reduce the size of the time zone database. tzdata and tz both work by compiling the IANA database to an internal format. At the time, tzdata compiled to a 3.5 MB ets table (~600 KB gzip compressed) and tz compiled to a 300 KB beam file (~250 KB gzip compressed). Using TZif files (the /usr/share/zoneinfo ones) and 10 years of time zone records for all time zones resulted in about 450 KB of data (~16 KB gzip compressed).



First, add nerves_time_zones to your list of dependencies in mix.exs:

def deps do
    {:nerves_time_zones, "~> 0.1.2"}

NervesTimeZones persists the currently selected local time zone to "/data/nerves_time_zones". This works well on Nerves devices. If you're developing on your laptop, you may want to change the location by adding the following in your project's config.exs:

config :nerves_time_zones, data_dir: "./tmp/nerves_time_zones"

The fetched IANA timezone db version defaults to 2022g, which can be adjusted like this:

config :nerves_time_zones, version: "2025a"

The default time zone is "Etc/UTC". If you want it to be something else, set it in the config like this:

config :nerves_time_zones, default_time_zone: "Europe/Paris"

NervesTimeZones maintains only a subset of the available timezone database information to save on file size. The default keeps [2022-01-01, +10 years] relative to the date of compilation. Both ends can be adjusted in the config like this:

config :nerves_time_zones,
  earliest_date: DateTime.to_unix(~U[2022-05-17 12:02:32Z]),
  latest_date: System.os_time(:second) + 5 * 365 * 86400


Database example

If you just start up IEx, you may have seen something like this:

{:error, :utc_only_time_zone_database}

NervesTimeZones automatically sets up the time zone database so once you've added the :nerves_time_zones dependency, you'll get this instead:

{:ok, #DateTime<2021-03-11 10:19:59.811175-05:00 EST America/New_York>}

NervesTimeZones is opinionated on the time zone database provider so it forces the default and will log messages if there's a conflict with tzdata or tz. You can still use those time zone databases if you really want them even though it defeats the purpose of keeping one database on a device. You'll just need to manually specify the database in all of your DateTime calls.


Local time example

By default with NervesTimeZones, local time will be UTC. You can see this by running NaiveDateTime.local_now/1. Be aware that this behavior is different from the normal behavior of using your system's local time zone setting if you trying this out on your laptop. Nerves devices don't have time zone settings by default.

iex> DateTime.utc_now
~U[2021-03-11 15:10:41.573579Z]
iex> NaiveDateTime.local_now
~N[2021-03-11 15:10:44]

You can set the time zone like this (note the time shift by 5 hours):

iex> NervesTimeZones.set_time_zone("America/New_York")
iex> NaiveDateTime.local_now
~N[2021-03-11 10:11:02]


Running OS commands

It's possible to use the same time zone database with non-BEAM programs. For example, on my system the default for C programs is Eastern time:

iex> System.cmd("date", [])
{"Thu 11 Mar 2021 10:34:14 AM EST\n", 0}

On a Nerves device, this would be UTC, but the concept is the same.

Say I want it to be Hawaii time:

iex> NervesTimeZones.set_time_zone("Pacific/Honolulu")

This won't affect the date program since it's not running on the BEAM. All is not lost. NervesTimeZones can provide environment settings so that the C runtime will use the same data base and time zone setting as on the BEAM:

iex> System.cmd("date", [], env: NervesTimeZones.tz_environment())
{"Thu 11 Mar 2021 05:40:38 AM HST\n", 0}
iex)> NaiveDateTime.local_now
~N[2021-03-11 05:40:49]


How it works

NervesTimeZones pulls data from the IANA time zone database and compiles it to TZif files using zic(8). This is the same process used to create the files under /usr/share/zoneinfo. The difference is that those contain time period records 50 years or more in the past and over 15 years to the future. NervesTimeZones limits the range substantially to reduce the database size.

Since the main embedded use cases for time zone information are to show the time and schedule events in the local time, having past time zone information is not needed. This saves a ton of space, since time zones changed a lot in the 20th century.

The second part of the library is a NIF that updates the C runtime's local time zone setting. This setting also affects Erlang's and Elixir's local time functions like NaiveDateTime.local_now/0 as well. The NIF is trivially short and calls tzset(3) with the TZif file. Unfortunately, the way to pass the time zone is via the TZ environment variable, so if TZ previously pointed to anything before this library runs, it won't afterwards.



Copyright (C) 2021 Frank Hunleth

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