nerves_time

NervesTime keeps the system clock on Nerves devices in sync when connected to the network and close to in sync when disconnected. It's especially useful for devices lacking a Battery-backed real-time clock and will advance the clock at startup to a reasonable guess.

Installation

First add nerves_time to your project's dependencies:

def deps do
  [
    {:nerves_time, "~> 0.4.2"}
  ]
end

Ensure that your vm.args allows for timewarps. If it doesn't, nerves_time will update the OS system time, but Erlang's system time will lag. The following line should be in the beginning or middle of the vm.args file:

+C multi_time_warp

If you're using one of the official Nerves Systems, then this is all that's needed. nerves_time requires Busybox's ntpd and date applets to be enabled. If you haven't explicitly disabled the, they're probably enabled.

Configuration

nerves_time uses ntp.pool.org for time synchronization. Please see their terms of use before tweaking nerves_time. Alternative NTP servers can be specified using the config.exs:

# config/config.exs

config :nerves_time, :servers, [
    "0.pool.ntp.org",
    "1.pool.ntp.org",
    "2.pool.ntp.org",
    "3.pool.ntp.org"
  ]

It's also possible to configure NTP servers at runtime. See NervesTime.set_ntp_servers/1.

nerves_time also has a concept of a valid time range. This minimizes time errors on systems without clocks or Internet connections or that may have some issue that causes a very wrong time value. The default valid time range is hardcoded and moves forward each release. It is not the build timestamp since that results in non-reproducible builds. Applications can override the valid range via the application config:

# config/config.exs

config :nerves_time, earliest_time: ~N[2019-10-04 00:00:00], latest_time: ~N[2022-01-01 00:00:00]

Algorithm

Here's the basic idea behind nerves_time:

If the clock hasn't been set or is invalid, set it to the earliest valid time known to nerves_time. This is either set in the application config or defaulted to a reasonable value that likely moves forward a little each nerves_time release.
check for time via a Real Time Clock
Run Busybox ntpd to synchronize time using the NTP protocol.
Update Real Time Clock periodically and on graceful power downs. This is currently only done at around 11 minute intervals.

To check the NTP synchronization status, call NervesTime.synchronized?/0.

Real Time Clock

A hardware based real time clock can be configured by added a config.exs entry:

config :nerves_time, rtc: {SomeImplementingModule, [some: :initialization_opt]}

By default Nerves Time is configured to use NervesTime.FileTime which will Check for ~/.nerves_time. If it exists, advance the clock to it's last modification time.

See the documentation for NervesTime.RealTimeClock to implement your own real time clock.

Credits and license