The cron rule that makes your job run 5x more often than you think

The five fields

Standard cron expressions have five fields: minute, hour, day of month, month, and day of week.

0 0 1 * 1

Why it runs more often

This expression means midnight on the first day of the month, or midnight every Monday. When both day-of-month and day-of-week are restricted, standard cron uses OR, not AND. In a month where the 1st is a Wednesday, it runs on the 1st and every Monday after it.

The first Monday

Standard cron cannot cleanly express “the first Monday of the month.” Put a day-of-week check inside the job, or use a scheduler with Quartz-style extensions.

AdroitPad's Cron Explainer shows the next six fire times and calls out this OR rule before you ship the schedule.