TimeRemaining will countdown if currentTime isn't directly passed to the component. Try it yourself by removing the prop and setting countDownEnd to the near future.

01d 00h 00m 00s

<TimeRemaining
  currentTime="2019-01-02T20:15:00.000-04:00"
  countdownEnd="2019-01-03T20:15:00.000-04:00"
  highlight="purple100"
/>

01d 00h 00m 00s

Custom label with time remaining

<TimeRemaining
  currentTime="2019-01-02T20:15:00.000-04:00"
  countdownEnd="2019-01-03T20:15:00.000-04:00"
  highlight="purple100"
  labelWithoutTimeRemaining="Custom label without time remaining"
  labelWithTimeRemaining="Custom label with time remaining"
  timerFontSize="4"
  labelFontSize="2"
/>