Add the package to your mix.exs dependencies:
{:clock, "~> 0.1"}
In your code, hold onto a clock struct and use the protocol for all things time related.
clock = %Clock.RealClock{}
...
now = Clock.now(clock)
In tests, make sure the clock struct is a FakeClock and use it to time travel.
clock = %Clock.FakeClock{}
...
now = Clock.now(clock)
Clock.FakeClock.travel(clock, 60_000)
one_minute_later = Clock.now(clock)