Convert Quartz to Unix cron

Lossy by nature

Unix cron has no seconds, no year, and none of Quartz’s L, W or # operators. The example — 18:00 on the last Friday of each month — has no crontab equivalent. CronViewer shows the closest expression, marks the conversion “not equivalent”, and gives the usual workaround: run every Friday at 18:00 and let the command exit unless next week is a different month. systemd OnCalendar can express it exactly (Fri *-*~07/1 18:00), so check that target too.

For expressions that do convert, weekday numbers are shifted back (Quartz 2-6 → Unix 1-5), “?” becomes “*”, and the equivalence check confirms the result. Seconds other than 0 and specific years are reported as lossy rather than silently dropped.