Your cron expression can be valid and still never run

Your cron expression can be valid and still never run A cron parser can answer one question— does this have five fields and legal tokens? —while your scheduler needs a different answer: will this job ever run, and will it run when I intended? That gap is where silent cron failures live. A schedule can be syntactically valid, return no parser error, and still be impossible, surprisingly broad, or operationally noisy. Here is a small semantic checklist you can run before deploying a schedule. 1. Check the calendar, not only the grammar Consider: 0 0 30 2 * This has the right five-field shape: minute, hour, day of month, month, day of week. But February has no day 30. A syntax-only validator can label it valid, while a calendar-aware validator should tell you that its approximate frequency is never. That distinction is important in CI: treat a parse error as a broken input, but treat an impossible calendar match as a review failure. Both deserve attention, but they need different messages. The same idea applies to leap days. 0 0 29 2 * is meaningful, but it does not fire in non-leap years. Whether that is correct depends on the job; the validator should surface the edge case instead of silently deciding for you. 2. Be explicit about day-of-month/day-of-week semantics This expression is a classic source of surprises: 0 0 1,15 * 1 Many traditional cron implementations treat a restricted day-of-month and a restricted day-of-week as an OR , not an AND. In that model, the job runs on the 1st, the 15th, or Monday. Someone reading the expression as “the 1st or 15th when it is Monday” will get a different schedule. This is not a universal rule across every scheduler, so check the documentation for the runtime that will execute the job. The useful validation behavior is to warn whenever both fields are restricted and force the author to choose the intended semantics. For example, a lightweight pre-deployment check can start like this: function semanticWarnings ( expression ) {