useIsOnShift.js

const lastShiftSelector = state => state.shifts.slice(-1)[0]

const useIsOnShift = () => {
  const lastShift = useSelector(lastShiftSelector)
  const currentTime = useCurrentTime()

  const [isOnShift, setIsOnShift] = useState(false)

  useEffect(() => {
    if (lastShift) {
      setIsOnShift(currentTime.isBefore(lastShift.endTime))
    }
  }, [currentTime])

  return isOnShift
}