Cron Jobs Explained: How to Read, Write, and Avoid Common Scheduling Pitfalls

Cron is a time-based job scheduler originating in Unix systems since the 1970s, now widely used in tools like GitHub Actions, Kubernetes, and CI pipelines. A standard cron expression consists of five space-separated fields representing minute, hour, day-of-month, month, and day-of-week, with special characters enabling lists, ranges, and step intervals. A critical pitfall is that classic cron lacks timezone support, meaning jobs run in the host machine's timezone — typically UTC — which can cause schedules to shift during daylight saving time changes. Another common mistake is combining day-of-month and day-of-week fields, which cron evaluates with OR logic rather than AND, often producing unintended run times. Verifying any cron expression with a parser tool before deployment is strongly recommended to prevent silent failures in automated tasks.