Cron every 15 minutes: why */15 * * * * runs on the clock

Cron reads five fields, and this schedule only changes the first one. The minute field */15 is a step: it starts at the field's lowest value, 0, and counts up by 15 until the field runs out. The other four fields are all *, which matches every value, so the job runs in every hour, on every date, in every month and on every day of the week. Here's a full line that appends the date to a log on each run:

*/15 * * * * date +\%F >> /var/log/poll.log 2>&1

The % needs care. Cron treats an unescaped percent sign as a line break, and crontab(5) says a % "will be changed into newline characters" unless a backslash escapes it. Everything after the first one goes to the command as standard input, so date +%F never reaches date whole. The backslash in \%F keeps the percent sign literal. A script file doesn't need the escape, since the rule only applies to the crontab line.

The step starts over at the top of each hour, and it's the first trap to know. */15 is safe because 15 divides 60, so every hour repeats the same marks. */45 is not a 45 minute interval. It matches minutes 0, 45, so the run at :45 is followed by the run at the top of the next hour 15 minutes later. crontab(5) says steps "are evaluated just within the field they are applied to", which explains it. For a true 45 minute interval, cron is the wrong tool, and cron vs systemd timers compares the two.

The schedule is also clock aligned, and it keeps no memory. If the server is down at the :15 mark, that run is lost, and nothing comes back to run it once the server is up. If a missed run matters, the job has to cope with that. Have it save the time of its last successful run, then handle everything since then on its next run.

Cron also reads the server's time zone by default, so the :00, :15, :30 and :45 marks are server time, which may not match your own clock. Cron time zone pitfalls covers how to set the zone and what daylight saving time does to it.

The 5 minute schedule, */5 * * * *, runs more often than this one, 288 times a day against 96. The 30 minute schedule, */30 * * * *, runs less often, at 48 times a day. A job that runs longer than 5 minutes overlaps itself on the 5 minute schedule, but this one gives it 15 minutes before the next start.