public static Comparator<Period> comparingByFromTime() {
        return Comparator.comparing(Period::getFromMs);
    }
<p>
Class that represents a period. May have null upper or lower bounds to
indicate before or after a time rather than between 2 times.
</p>
<p>
<p>
The upper bound is not part of the period. E.g. 1/1/2001 to 1/1/2002 does not
include 1/1/2002.
</p>
stroom-util/src/main/java/stroom/util/Period.java