# `Rheo.Clock`
[🔗](https://github.com/thanos/rheo/blob/v1.0.0/lib/rheo/clock.ex#L1)

Injectable clock used for lease expiry and event timestamps.

Rheo reads time through this behaviour so tests can advance time without
`Process.sleep/1` via `Rheo.Clock.Frozen`.

## Callback

See `c:utc_now/0`.

## Example implementation

    defmodule MyApp.Clock do
      @behaviour Rheo.Clock

      @impl true
      def utc_now, do: DateTime.utc_now() |> DateTime.truncate(:millisecond)
    end

Configure with:

    config :rheo, clock: MyApp.Clock

# `utc_now`

```elixir
@callback utc_now() :: DateTime.t()
```

Returns the current UTC time.

## Examples

    defmodule Demo.Clock do
      @behaviour Rheo.Clock

      @impl true
      def utc_now, do: ~U[2026-01-15 12:00:00.000Z]
    end

## Returns

A `DateTime.t()` in UTC, typically truncated to milliseconds.

# `impl`

```elixir
@spec impl() :: module()
```

Returns the configured clock module.

Defaults to `Rheo.Clock.System` when `:clock` is unset.

## Examples

    iex> is_atom(Rheo.Clock.impl())
    true

## Returns

A module implementing `Rheo.Clock`.

# `utc_now`

```elixir
@spec utc_now() :: DateTime.t()
```

Current UTC time according to the configured clock.

## Examples

    iex> match?(%DateTime{}, Rheo.Clock.utc_now())
    true

## Returns

A `DateTime.t()`. Delegates to `c:utc_now/0` on `impl/0`.

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