GenServerで実装

stopwatchC.ex

defmodule StopwatchC do
  use GenServer
  
  import System, only: [ monotonic_time: 1 ]
  
  @gclock __MODULE__
  
  def activate() do
    now = monotonic_time(100)
    GenServer.start_link(StopwatchC, now, name: @gclock)
    :ok
  end

  @impl true
  def init(last_time) do
    {:ok, last_time}
  end

  @impl true
  def handle_call(:restart, _from, last_time) do
    now = monotonic_time(100)
    {:reply, now - last_time, now}
  end
  
  @impl true
  def handle_call(:lap, _from, last_time) do
    now = monotonic_time(100)
    {:reply, now - last_time, last_time}
  end
  
  def restart() do
    GenServer.call(@gclock, :restart)
  end
  
  def lap() do
    GenServer.call(@gclock, :lap)
  end
end