public static Instant durationToThreshold(final Instant now, final StroomDuration stroomDuration) {
        return durationToThreshold(now, stroomDuration.getDuration());
    }
Converts a duration, e.g. 30days into a point in time in the past, i.e.
current system time minus the duration.

@param now The instant used to represent the current time. Useful for testing purposes.
stroom-util/src/main/java/stroom/util/time/TimeUtils.java