I am trying to add a crontab entry to execute a script every 30 minutes, on the hour and 30 minutes past the hour or something similar. How can I do this?

Cron is matching, not counting. 30 * * * * says “the minute is 30” — and the wildcard hour says “any hour”. The result is twenty-four runs per day, all at half past.

To run twice an hour (e.g. on the hour and at 30 past), use a list: 0,30 * * * *. Or step syntax: */30 * * * * (functionally identical, expands to 0,30). For a different cadence — say every 20 minutes — use */20 * * * * for 00/20/40 or 10-50/20 * * * * for 10/30/50.

One frequent confusion: people expect */30 * * * * to “sleep 30 minutes” between runs. It does — but only because 00 and 30 happen to be exactly 30 minutes apart inside one hour. The idea of an interval is implicit; cron itself just matches the minute field. If your schedule needs to be measured from the previous run (e.g. 30 minutes after job ends, regardless of clock time), use a job runner with that semantic, not cron.