static set now(n) {
    now = n;
  }
Set the callback for returning the current timestamp.
The function should return a number, which will be interpreted as an Epoch millisecond count
@type {function}
@example Settings.now = () => Date.now() + 3000 // pretend it is 3 seconds in the future
@example Settings.now = () => 0 // always pretend it's Jan 1, 1970 at midnight in UTC time