Cron steps are calendar selections, not elapsed-time timers

A step applies within a field

A minute expression */15 selects minutes 0, 15, 30 and 45 in each hour. It is evaluated against the calendar field.

This looks like a regular 15-minute interval, but the rule comes from field values, not a timer started at the deployment instant.

*/15 * * * *
Minutes: 00, 15, 30, 45

Nondividing steps reveal the boundary

A minute expression */40 selects minute 0 and 40. The gap from 00:40 to 01:00 is 20 minutes, not 40.

Use a short preview across the hour boundary to show this. A fixed every-40-minutes requirement needs an elapsed-time scheduling mechanism.

*/40 * * * *
00:00 → 00:40 → 01:00 → 01:40

Months have variable duration

Running on the first of each month is a calendar rule. Month lengths differ, so it is not a fixed 30-day recurrence.

Similarly, a local daily job can have a 23- or 25-hour gap near daylight-saving transitions. Decide which meaning the business actually needs.

Check several boundaries

Preview an hour boundary, month boundary and clock change as relevant. Compare actual intervals if the requirement is described as elapsed time.

Use a durable timer or queue when elapsed duration is required. Cron should express a calendar schedule rather than approximate an undocumented timer.