# `NestGen2.Clock`
[🔗](https://github.com/integration-technology/nest_gen2_sdk/blob/v0.6.6/lib/nest_gen2/clock.ex#L1)

Keeps the system clock (UTC) right.

The Nest restores UTC from its battery-backed RTC at boot, but nothing
corrects it afterwards: Nest's own NTP client lived in `connmand`, which
isn't reliable once the stock stack is off. This asks NTP servers (SNTP over
UDP 123) shortly after start and then every `:clock_sync_interval_ms`,
steps the system clock when it is out by `:clock_step_ms` or more, and saves
it to the RTC after every successful check so the next boot starts right.

Servers are tried in order from `:time_servers` (host names, IPv4 tuples, or
`:gateway` for the default router).

Apps that already talk to a trusted HTTPS server can pass its `Date` header
to `observe/2` as a fallback: it only corrects the clock when NTP hasn't
succeeded for 12 hours and the difference is over 30 seconds.

Local time and daylight saving are the app's concern; this only keeps UTC.
The VM must run with `+C multi_time_warp` (as `app_watchdog.sh` does) so
Erlang's own time follows a stepped clock.

# `child_spec`

Returns a specification to start this module under a supervisor.

See `Supervisor`.

# `observe`

```elixir
@spec observe(DateTime.t(), term()) :: :ok
```

Reports the UTC time according to another source (e.g. an HTTPS `Date`
header). Used only as a fallback when NTP has been failing.

# `start_link`

# `status`

```elixir
@spec status() :: map()
```

When and how the clock was last checked.

# `sync`

```elixir
@spec sync() :: {:ok, %{server: term(), offset_ms: integer()}} | {:error, term()}
```

Queries NTP now and corrects the clock if needed.

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*Consult [api-reference.md](api-reference.md) for complete listing*