Cron expression explainer

Reads a five-field cron expression and says what it means in plain English, validating each field’s range as it goes.

The single most misunderstood thing about cron is what happens when both day fields are set. `0 0 1 * 1` looks like "the first of the month, if it is a Monday" and actually means "the first of the month, OR any Monday" — cron ORs the two day fields rather than ANDing them, so that expression runs far more often than intended. This flags it explicitly. The other regular confusion is field count: the standard Unix format is five fields, and the six-field form with seconds is Quartz, used by Java schedulers and Spring. Pasting one into the other is a common and silent failure.