All Cron guides
Newest first. Each one names the runtime its outputs came from.
Cron Field Count — A Six-Field Expression Still Installs, and Runs Wrong
crontab reads five fields and calls the rest the command, so a six-field Spring expression installs and runs on a different schedule entirely.
Cron vs systemd Timers — What You Get for the Extra Files
Two files instead of one line, in exchange for logging, missed-run catch-up and arbitrary intervals.
Cron and Timezones — The Two Days a Year It Goes Wrong
Local time means an hour vanishes each spring and repeats each autumn. CRON_TZ and the UTC fix.
Cron Every 5 Minutes — And Why */45 Does Not Do What You Think
*/5 works, but a step that does not divide 60 restarts hourly — */45 fires 45 minutes apart, then 15.
Crontab Syntax — The Five Fields, Explained with Real Run Times
The five fields, what each accepts, step-value semantics, and common expressions with their real next run times.
Cron Day of Week — Is Sunday 0 or 7, and the OR Rule
Both 0 and 7 mean Sunday, Quartz numbers it 1, and the OR rule that makes jobs run far more often than intended.