About CronLens
CronLens explains cron expressions and lists when they will run next, for the five places people most often meet them: Linux cron, Kubernetes CronJobs, GitHub Actions, the Quartz scheduler and AWS EventBridge.
Why it exists
Cron looks like one language, but it is a family of dialects. The same five or six fields mean different things
depending on who reads them: Quartz and AWS require a ? in one day field, Quartz starts with seconds, AWS ends with a year,
Sunday is 0 in one system and 1 in another, and what happens when clocks change is handled differently in nearly every
scheduler. A tool that guesses one dialect for everything gives confident wrong answers. CronLens makes you pick the dialect, follows that
system's own rules, and tells you where the rules are undocumented.
Everything runs in your browser
The parser, the explanation and the next-run calculation are TypeScript running on your device. The expression you type is not sent to a server. Pages are static files; there is no back end that receives your input. The details, including the two values the site stores in your browser's local storage, are in the Privacy Policy.
How the results were checked
- Hand-derived tests. The expected run times in the automated tests and in the worked examples on every page were worked out from the calendar and the documentation first (for instance, that 1 October 2026 is a Thursday and that the third Friday of October 2026 is the 16th), and only then compared with the code. The pages print those same checked values.
- Reference implementations. The Kubernetes engine was compared with robfig/cron v3.0.1, the Go library Kubernetes uses, and the Quartz engine with the Quartz 2.3.2 library, on randomly generated schedules including several time zones with daylight saving. No differences remained after the comparison found and we fixed some. The comparison cannot cover every rare combination, and one Quartz feature (nearest weekday after the 28th, in short months) is inconsistent in Quartz itself, so the tool says so.
- Documentation only. Linux cron, GitHub Actions and AWS EventBridge were checked against their documentation, not against the running services. We did not call AWS or GitHub, and the Linux daylight saving behaviour is taken from the manual pages, which do not fully agree with each other.
Where it is unsure
Each dialect page lists its limits, and the daylight saving note under every result says whether a behaviour is documented by the vendor, observed by running the real library, or unclear. Every page ends with the official documents it relied on, with links and the date they were read. If a source changes or you find a result that disagrees with your real scheduler, please tell us with the expression, the dialect, the time zone and what you saw; that is the most useful kind of correction.
Who runs it
CronLens, an independent project operated from Indonesia. You can reach us by email at joe@zona13.net.
Cost and advertising
The site is free. It currently shows no advertising and sets no advertising cookies. If that ever changes, the Privacy Policy and this page will be updated before it does.
What it is not
It is not a scheduler, does not run your jobs, and does not connect to your cluster, repository or cloud account. Treat its output as a
well-checked second opinion and confirm anything important in the system that will actually run the job, for example with a manual
workflow_dispatch run or a test CronJob. See the terms and disclaimer.