Cron Expression Explainer

In short A cron expression has five fields in a fixed order — minute, hour, day of month, month, day of week — and the errors people make are almost always in the same three places. The first is forgetting that omitting a field is not allowed: `0 2 * * *` runs at 02:00 every day, and leaving out the trailing asterisks changes the meaning rather than defaulting sensibly. The second is the day-of-month and day-of-week interaction, which is genuinely counter-intuitive: when BOTH are restricted, standard cron runs the job when EITHER matches, not both — so `0 0 13 * 5` fires on the 13th and on every Friday, not only on Friday the 13th. The third is step syntax, which counts from the start of the field's range rather than from the last run. `*/15` in the minute field gives 0, 15, 30 and 45, which is what people expect. `*/5` in the HOUR field gives 0, 5, 10, 15 and 20 — and then the day ends, so the gap between the 20:00 run and the next one at midnight is four hours rather than five. Steps do not wrap. Read the expression back in plain English before deploying it, because a cron that fires on the wrong days is usually discovered by its absence.