The cron hour field: 0 to 23 on a 24 hour clock

Cron thinks entirely in a 24 hour clock. The hour field is an integer from 0 to 23 and there is no notation for AM or PM, so every afternoon time has to be converted before it goes in the crontab. The absence of a suffix means a wrong hour is a valid expression, which is why this field produces more silent bugs than any other.

Midnight is 0 and noon is 12. That is the inverse of the twelve hour clock convention where midnight is written 12 AM, and the confusion produces a specific recurring bug: a job meant for midday scheduled at hour 0 and running twelve hours early every day for months before anybody notices.

Hour 24 does not exist. An expression using it is rejected as out of range rather than being treated as the end of the day, which is at least a loud failure. The end of a day in cron is 23:59, and a job meant to run at the very end of a day is usually written as 59 23 * * *.

Because the hour field is matched against local wall clock time, it is the field that daylight saving affects. In a zone that observes it, one hour a year does not exist and one hour a year happens twice, so an hour pinned schedule can be skipped or repeated. Running the daemon in UTC removes the question entirely.