A schedule was a time of day on a set of days, and that is still the form most people fill in. Two more shapes join it.
Every so many minutes or hours
Where to find it. Mac: Settings → The room → Schedule → Add one, the at a time menu, set to every…. Web: Settings → Schedule → Add one, the same menu. iPhone: New schedule → When → Repeat.
Type a number and choose minutes or hours. Slots are counted from midnight in your zone, so "every 90 minutes" runs at 00:00, 01:30, 03:00 … and starts again at midnight; a day never ends mid-interval. On our servers the shortest interval is five minutes; a Mac may go once a minute. The row reads Every 15 minutes or Every 6 hours, with when it next runs.
A cron line
Choose cron line and type five fields: minute, hour, day of the month, month, day of the week. Numbers, *, ranges (1-5), steps (*/15) and lists (1,15,30); 0 and 7 are both Sunday. Names (MON, JAN), L, W and ? are not read: a line that needs them is a line nobody can read back. When both day fields are restricted, a date matches if either does, as cron has always had it. A line that cannot be read, or that never comes round (0 0 31 2 *), is refused with the reason before it is kept; on our servers a line finer than every five minutes is refused too.
Lines worth knowing: */15 * * * * every quarter hour; 0 */6 * * * every six hours on the hour; 0 7 * * 1-5 weekdays at 07:00; 0 9 1 * * the first of the month at 09:00; */5 9-17 * * 1-5 every five minutes in business hours. The row says the plain ones as a person would and shows the others as the line.
What a missed slot means
One slot, never one per missed interval. A Mac asleep for an afternoon owes "every 15 minutes" one run when it wakes, not twelve; with catch up a missed run off, a slot more than an hour gone is let go. Our servers make up a slot within a day of it, once. A slot's run is recorded by its minute, so the same slot is never run twice, however many servers tick.
When not to use it. Watching one thing closely for an hour is a loop, which keeps one discussion and its memory; a cadence starts a fresh discussion each time.
Further reading. crontab — The Open Group, POSIX.1-2024.