Cron every 30 minutes: */30 * * * *

Half hourly is the schedule people reach for when hourly is not often enough and quarter hourly feels excessive. It fires on minute 0 and minute 30, the two busiest minutes on almost any machine, because they are also where hourly jobs, quarter hourly jobs and most human scheduled work land.

Since only two minutes are involved, the offset fix is unusually cheap: 15,45 * * * * gives the same two runs an hour on minutes that nothing else contends for. There is no step form that produces 15 and 45 from a bare asterisk, so the list is the natural way to write it, and 15-59/30 is a less readable way of saying the same thing.

A half hourly job has thirty minutes of headroom, which tempts people into doing real work in it: a full table scan, a report render, an export to object storage. That is fine until the work grows past the interval, and unlike a one minute job the failure is slow to appear. Log the duration of every run and alert when it passes half the interval, not when it passes the whole one.

The schedule takes no notice of the date, so it also fires forty eight times on the day of a planned maintenance window and forty eight times on Christmas Day. If the downstream system has a quiet period, adding an hour or day of week restriction to the same expression is a one field change and cuts a lot of pointless error mail.