Take a line from a crontab and find out when events will occur
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.