In addition to what einsty said (which is 100% accurate), if you're deleting aged records, on any system of sufficient size objects will become aged beyond your threshold between executions.
Right. You can kind of consider the state of a filesystem on which you occasionally run rm * purges to be a system whose state is made up of ‘stuff in the filesystem’ and ‘timestamp the last purge was run’.
If you run rm * multiple times, the state of the system changes each time because that ‘timestamp’ ends up being different each time.
But if instead you run an rm on files older than a fixed timestamp, multiple times, the resulting filesystem is idempotent with respect to that operation, because the timestamp ends up set to the same value, and the filesystem in every case contains all the files added later than that timestamp.
If you run rm * multiple times, the state of the system changes each time because that ‘timestamp’ ends up being different each time.
But if instead you run an rm on files older than a fixed timestamp, multiple times, the resulting filesystem is idempotent with respect to that operation, because the timestamp ends up set to the same value, and the filesystem in every case contains all the files added later than that timestamp.