Step larger than the field

A step like */15 means "every 15th value, counting from the start of the field". The catch is that counting never leaves the field. The minute field runs 0–59, so */90 starts at 0, tries to add 90, runs off the end of the range, and stops — leaving a single value, minute 0. It does not mean "every 90 minutes", because cron has no way to carry from minutes into the next hour.

This is flagged as an error rather than a warning because the expression almost certainly does not do what its author believed, and the true behaviour (fire once, at minute 0) is rarely the intent. A step of 0 is likewise invalid — you cannot select "every 0th value".

Each field has its own ceiling: 60 values for minutes and seconds, 24 for hours, 31 for day-of-month, 12 for months, 7 for day-of-week. A step at or above that ceiling collapses to the field's starting value, and a step above it is meaningless.

Intervals that span more than one field have to be expressed in the larger field. "Every 90 minutes" does not divide evenly into an hour and cannot be written as a single standard cron line at all — the closest clean options are 0 */2 * * * (every two hours) or two entries. "Every three hours" belongs in the hour field: 0 */3 * * * fires at 00:00, 03:00, 06:00 and so on. The rule of thumb: keep each step within its field's range, and push longer intervals up to the next field.