Cron Builder & Expression Reader
Build a cron schedule from dropdowns, or paste one you already have to see what it means and when it next fires. Outputs crontab, systemd, k8s and Actions.
Cron Builder & Expression Reader
No day of the week is ticked, so that field stays * — this schedule runs every day, not weekly.
Hand-edited, so the dropdowns above no longer describe this expression.
How to use Cron Builder
Build one, or read one
Two modes, same outputs. Build one starts from what you want to schedule and writes the expression. Read one takes an expression you already have — pasted from a crontab, a workflow file or a code review — and tells you what it means and when it fires next. Shorthands like @daily and @reboot are understood, as are names like MON-FRI and JAN.
Set the exact time — or edit a field directly
For daily/weekly/monthly schedules, pick the hour and minute (24-hour clock). For weekly, also tap the day-of-week pills — multiple days are allowed. For monthly, choose day-of-month; for yearly, pick the month too. The five boxes underneath show what those choices produced, field by field, and you can type straight into them in either mode — useful for the things no dropdown offers, like */3 hours or 1-5 day-of-month. Do that and the dropdowns stop driving (they cannot express most hand edits, so the tool says so rather than guessing); one click rebuilds from them again.
Paste your command
The command goes into the rendered crontab line and the example k8s/GitHub Actions output. Leave it as the placeholder if you only want the cron expression.
Check the next five runs, then copy
Before you copy anything, read the five fire times. That is the check that catches the mistakes a plain-English summary can hide — a day-of-week off by one, a 31st that skips February and April, a schedule that can never fire at all. Then click Copy on whichever of the four formats you need: bare crontab, systemd timer pair, Kubernetes CronJob YAML, or GitHub Actions workflow snippet. Share link copies a URL that reopens this page with the same expression loaded — useful for a pull-request comment or a ticket, where the reviewer gets the schedule and the fire times rather than five fields to decode.
Cron — still the world's most important scheduler
Cron has been Unix's standard task scheduler since 1975, when Brian Kernighan wrote the first version at Bell Labs. The five-field syntax — minute, hour, day-of-month, month, day-of-week — is identical in 2026 to what it was 50 years ago. Modern alternatives layered on top (systemd timers, Kubernetes CronJob, GitHub Actions schedules, AWS EventBridge rules) all use cron expressions as their input format, because nothing simpler has ever caught on at the same scale. If you write code that needs to run on a schedule, you will eventually write a cron expression — usually at the worst possible moment, often in production, frequently while wondering whether asterisk-slash-15 means "every 15 minutes" or "for 15 minutes".
Why a visual builder beats writing it by hand
Most cron bugs come from the same three confusions: forgetting that day-of-week starts at Sunday=0 (not Monday=1 like every calendar humans use), writing * * 1 * 1 when you meant "first Monday of the month" (it actually runs every Monday and the 1st of every month), and mistaking the comma operator for a range. A wizard sidesteps all three by translating your intent into the right expression. Power users still prefer to type it directly — so Read one takes the typed expression instead and answers the question a wizard cannot: not "what did I mean" but "what does this line, already in production, actually do". Either way the five fire times are the real check. A summary can describe 0 9 1 * 1 accurately and still leave you thinking it means the first Monday of the month; a list showing the 1st, then the 2nd, then the 9th does not.
The cron expression for "every day at 9am" is0 9 * * *. The cron expression for "every weekday at 9am" is0 9 * * 1-5. Get the day-of-week field wrong and you discover the bug at 09:01 on Saturday.
Cron in modern infrastructure
Plain crontab is fine for a single-server VPS or a shared host where you can SSH in. systemd timers have largely replaced it on modern Linux distros — they survive missed runs (the Persistent=true option), integrate with the journal for logging, and can be unit-tested in isolation. Kubernetes CronJob handles scheduling across a cluster, manages pod lifecycle, and provides retries via backoffLimit. GitHub Actions schedules run inside CI/CD without needing infrastructure of your own — but be warned: GitHub's cron only fires once every 5 minutes minimum, runs in UTC regardless of your repo timezone, and is sometimes delayed by up to 30 minutes during high-traffic periods.
The APAC scheduling landscape
Scheduling decisions across APAC infrastructure mirror the global trends but with some local nuance. Singapore, Hong Kong, Tokyo, Sydney are AWS heartland markets — most scheduled jobs run on EventBridge Scheduler or ECS scheduled tasks, both of which take cron syntax. Jakarta and Mumbai are the fastest-growing AWS regions; Indonesia and India teams are also heavy users of Google Cloud Scheduler (also cron syntax). South Korea has strong Naver Cloud Platform adoption alongside AWS. China uses Alibaba Cloud's Container Service for Kubernetes (ACK), with CronJob resources identical to upstream k8s, plus Function Compute's own time-trigger cron. Across the region, timezone handling is the single most common bug — Singapore, Malaysia, Philippines, China, and Hong Kong all sit at +8 hours, but a job scheduled "for 09:00" without timezone will fire at 17:00 local because the cluster defaults to UTC. Always set timezone explicitly (spec.timeZone: "Asia/Singapore" in modern k8s, TZ=Asia/Singapore in crontab) — and re-check after every cluster upgrade because the default has shifted between versions.
Things cron does not handle well
Cron does not retry on failure (your script has to handle that). It does not track whether the previous run is still in progress (use a lockfile or systemd's OnUnitInactiveSec= instead). It cannot fire on an interval like "every 90 minutes" — the field is minute or hour, not multiples in between. For anything more complex than a fixed schedule, look at task queues (Celery, Sidekiq, Laravel queues) backed by Redis or a database; those handle retries, distributed locking, and observability in ways that no version of cron ever will.
10 Things You Didn't Know About Cron
The name "cron" comes from the Greek word chronos (time). Brian Kernighan wrote the original at Bell Labs in 1975.
The five-field syntax has not changed in 50 years — making cron one of the longest-lived APIs in computing history.
Day-of-week numbering varies: classic Unix cron uses Sunday=0, but ISO 8601 uses Monday=1. Most modern crons accept both 0 and 7 for Sunday.
When both day-of-month AND day-of-week are specified, classic cron runs when EITHER matches — not when both match. This trips up almost everyone.
GitHub Actions cron schedules are not guaranteed — they can be delayed up to 30 minutes during high-traffic periods, and skipped entirely during platform incidents.
Kubernetes CronJob added timezone support in v1.27 (2023). Before that, cluster UTC was the only option — causing endless "why is my job running at midnight" tickets.
systemd timers can fire with millisecond precision using OnCalendar=*-*-* *:*:00/0.5. Classic cron's minimum interval is 1 minute.
AWS EventBridge Scheduler supports cron AND rate expressions ("every 5 minutes") plus one-time scheduled events — extending classic cron's vocabulary.
The string "@reboot" in crontab runs a command at system startup. "@daily", "@hourly", "@weekly", "@monthly", and "@yearly" are all valid shorthands.
Alibaba Cloud Function Compute's time-trigger uses a 7-field cron — adding seconds and year to the standard 5 fields, like AWS Lambda's pattern.
FAQ
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Yes — switch to Read one and paste it. You get a plain-English summary, the next five fire times in your own timezone, and a specific error if it will not parse (which field, and what that field accepts). The five per-field boxes stay in sync with the expression in both modes, so you can change just the day-of-week and watch the fire times move. Shorthands (
@daily,@hourly,@reboot), names (MON-FRI,JAN) and7for Sunday are all understood. Six- and seven-field expressions — the Quartz, AWS EventBridge and Alibaba Function Compute dialects, which add seconds and/or year — are rejected with a note saying so rather than silently misread. -
Share link copies a URL with the expression in it —
?e=0+9+*+*+1-5— which reopens the page with that schedule loaded, explained, and showing its next five runs. The link is deliberately readable rather than an encoded blob, so it still says what it does when someone reads it in a pull request, and it can be edited by hand. On a phone it opens the native share sheet instead of copying. Nothing is stored on our side: the schedule travels in the link, and the address bar tracks your current expression so a refresh or a bookmark keeps it. -
Classic cron uses the server's local timezone (set by
/etc/timezone). Kubernetes CronJob defaults to UTC unless you setspec.timeZone(v1.27+). GitHub Actions always uses UTC and ignores any timezone hints. Always verify by running the job once and checking the actual fire time. -
The 5-field expression works on standard Unix cron, Linux crontab, systemd
OnCalendar(with format adjustment, shown in the systemd output), Kubernetes CronJob, GitHub Actions, GitLab CI schedules, and most cloud schedulers. AWS Lambda, Google Cloud Scheduler, and Alibaba Function Compute use a 6- or 7-field variant — use their dedicated tools for those. -
It's the "step" operator.
*/15in the minute field means "every 15 minutes" (i.e. 0, 15, 30, 45). It does NOT mean "for 15 minutes". The starting offset is 0 unless you write a range like5-55/15. -
Not directly — cron's step values must divide their field evenly (60 / N = whole number for minutes). "Every 90 minutes" would need two entries: one for minute 0 every other hour, one for minute 30 in between. Easier to use systemd timers (
OnUnitActiveSec=90min) or a task queue. -
Runs the command once at system startup. Not a true cron schedule but a useful shortcut for boot-time scripts. Not supported in Kubernetes CronJob or GitHub Actions — use the platform's native startup hooks instead.
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Cron itself has no overlap protection. Wrap the command in
flock -n /tmp/myjob.lock --to use a filesystem lock. systemd timers can useOnUnitInactiveSec=instead ofOnCalendar=to chain runs. Kubernetes CronJob hasconcurrencyPolicy: Forbid. -
GitHub explicitly documents that scheduled workflows can be delayed by up to 30 minutes during high-traffic periods (start of hour is the worst — millions of jobs queue at the same moment). Avoid scheduling at :00 if possible — :07, :13, :23 spread the load. Critical jobs should run elsewhere.
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Cron for simple, fixed-schedule tasks that don't need retries or visibility (rotate logs, backup database). Task queue (Celery, Sidekiq, Laravel queues backed by Redis) for anything that needs retries on failure, distributed locking, observability, or fire-and-forget jobs triggered by user actions.
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For classic cron: just run the command directly in a shell. For systemd timers:
systemctl start myjob.servicetriggers an immediate run. For Kubernetes CronJob:kubectl create job --from=cronjob/myjob myjob-manual. For GitHub Actions: use theworkflow_dispatchtrigger to add a manual-run button.
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