A cron schedule: a 5-field expression (minute hour day month weekday) or a nickname.
Nicknames: @yearly, @annually, @monthly, @weekly, @daily, @midnight, @hourly.
Fields support *, numbers, ranges (1-5), steps (1-30/2), comma lists (1,5,10), and month/weekday names (JAN-DEC, SUN-SAT).
Validated at runtime by the cron parser.