Every backend eventually grows a cron job. Backups at 2 AM, digests at 8 AM, health checks every five minutes. And every team eventually hits the same wall: the schedule that fires at a time nobody asked for, in a timezone nobody configured, on a day-of-week nobody intended. Cron syntax is five fields. It fits on a sticky note. The bugs do not come from the syntax — they come from the three places where different implementations disagree with each other. This is a field guide to all of it. ...