Three dialects
Unix is five fields. Quartz is six or seven (seconds first, optional year). AWS EventBridge is six fields from minutes through year, often wrapped in `cron()`.
Why `?` exists
Quartz and AWS cannot OR day-of-month and day-of-week. The unused field is `?`. Unix uses `*` and ORs when both are set.
Seconds and year
Unix has neither. Seconds 0 drop with a note. Sub-minute seconds block Unix rather than inventing `*`. Non-wildcard years warn and drop.
Unix OR vs exclusive fields
If Unix pins both DOM and DOW, you must choose which constraint to keep for Quartz/AWS.
CronFox stores Unix only
Deploy uses the Unix projection. The Quartz or AWS source is never saved on a task.