Given a line from a crontab, tells you the time at which cron will
next run the line, or when the last event occurred, relative to any
date you choose. The object keeps that reference date internally,
and updates it when you call nextEvent() or previousEvent() - such
that successive calls will give you a sequence of events going
forward, or backwards, in time.

Use setCounterToNow() to reset this reference time to the current
date on your system, or use setCounterToDate() to set the reference
to any arbitrary time, or resetCounter() to take the object back
to the date you constructed it with.

This module uses Set::Crontab to understand the date specification,
so we should be able to handle all forms of cron entries.