Returns a time mark on the same time source that is behind this time mark by the specified duration.
If the time mark is adjusted too far in the past or in the future, it may saturate to an infinitely distant time mark. In that case, elapsedNow will return an infinite duration elapsed from such infinitely distant mark.
IllegalArgumentException - an implementation may throw if a positive infinite duration is subtracted from an infinitely distant future time mark or
a negative infinite duration is subtracted from an infinitely distant past time mark.