Copying cron expressions across schedulers can silently break your job schedule

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A team migrating a job from Quartz to Unix crontab copied the day-of-week range 2-6 directly, not realising the two systems number weekdays differently. Quartz treats Sunday as 1, making 2-6 Monday through Friday, while Unix cron treats Sunday as 0, making the same range Tuesday through Saturday. The job silently ran on the wrong days — skipping Monday and firing on Saturday — with no errors or crashes to signal the problem. The root cause is the lack of a unified cron standard: at least eight dialects are in common use, differing on field count, Sunday numbering, alias support, and special tokens. Developers porting cron expressions between systems such as Quartz, Vixie cron, AWS EventBridge, Kubernetes, and GitHub Actions must manually verify day-of-week numbering and syntax rules for each target platform.

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