Cron Expression Generator & Explainer

A cron expression is the most compact way to schedule recurring work in Unix-like systems, but its terse five-or-six field syntax is famously hard to read back, especially a year after you wrote it. This tool parses any cron expression you type, computes the next 10 to 20 fire times in your chosen IANA timezone, and writes out a plain-English description like "Runs at 09:30 on Monday" or "Runs every 5 minutes" so you (and your on-call colleague) can verify the schedule at a glance.

Both flavours are supported: the traditional 5-field POSIX layout (minute, hour, day-of-month, month, day-of-week) used by Vixie cron, systemd timers and most cloud schedulers, and the 6-field Quartz layout used by Java's org.quartz.CronExpression, Spring @Scheduled, and many job-orchestration libraries, the only difference is an optional leading seconds field that gives sub-minute resolution.

How to Use

1. Pick your timezone, the dropdown lists the most common IANA names (UTC plus the Americas, Europe, Africa, Asia, Australia and Pacific zones). The computed fire times are anchored to this zone's wall clock, not the server's local time.
2. Type the cron expression in the main field. Wildcards (*), lists (1,15), ranges (9-17), steps (STAR/5), and Quartz ? are all accepted. Day-of-week and month also accept three-letter names (MON, JAN). Aliases like @daily and @reboot are recognised and expanded.
3. Choose how many fire times to show, 5, 10, 15 or 20 next fires. The table updates instantly.
4. Read the result, the canonical (5- or 6-field) expression appears in the badge, the plain-English summary just below it, and the list of upcoming runs shows each date in the chosen timezone plus a small "in 4 h" / "in 2 d" indicator.
5. Browse the presets for one-click examples (every minute, weekdays at 08:30, Quartz every 30 seconds, year-end fire, …). Click a preset to populate the field.
6. Open the field reference for the syntax cheat sheet, field ranges, special characters, and the canonical @yearly / @monthly / @weekly / @daily / @hourly shortcuts.

The Formula (Anatomy of a Cron Expression)

There is no single "formula" behind cron; instead the engine evaluates every wall-clock instant against six independent range checks. The two accepted column orders are:

5-field POSIX:   MINUTE  HOUR  DAY-OF-MONTH  MONTH  DAY-OF-WEEK
6-field Quartz:  SECOND  MINUTE  HOUR  DAY-OF-MONTH  MONTH  DAY-OF-WEEK

Allowed operators in every field:

Field ranges: second 0 to 59, minute 0 to 59, hour 0 to 23, day-of-month 1 to 31, month 1 to 12 (or JAN..DEC), day-of-week 0 to 6 (or SUN..SAT; POSIX accepts 7 to mean Sunday). Both DOM and DOW may match names: MON, TUE, …, JAN, FEB, …

The DOM/DOW rule (the most-common source of bugs): when both fields are restricted (neither is *), most cron daemons use OR semantics, the job fires if EITHER the day-of-month OR the day-of-week matches. So 0 0 1 * 1 means "00:00 on the 1st of every month OR every Monday", not "00:00 on the first Monday of the month." If you want one constraint only, set the other to *.

The Quartz DOM/DOW rule is stricter: at least one of them must be ? (the "no-spec" marker) to disambiguate, otherwise the expression is invalid. This tool accepts ? and treats it as * for forward compatibility.

Worked Examples

Example 1, 0 9 * * 1-5 (weekdays at 09:00): the description reads "Runs at 09:00 on Monday, Tuesday, Wednesday, Thursday, Friday." The next-fire table shows the next 10 weekday 09:00 instants in the chosen timezone.

Example 2, STAR / 5 * * * * (every 5 minutes): description "Runs every 5 minutes." The next-fire table lists every wall-clock minute that is a multiple of 5, in step order.

Example 3, 30 14 1 * * (1st of each month at 14:30): description "Runs at 14:30 on day 1 of the month." Demonstrates the DOM-only constraint cleanly (because DOW is *).

Example 4, 0 0 1 1 * (midnight on 1 January): description "Runs at 00:00 on day 1 of the month in January." Just one fire per year.

Example 5, STAR / 10 9-17 * * * (every 10 min, business hours): description "Runs every 10 minutes of every hour." The next-fire table shows a contiguous wall-clock streak during 09:00 to 17:50 with no entries overnight.

Example 6, Quartz 0 15 10 ? * MON (every Monday 10:15:00): description "Runs at 10:15 on Monday." Note the leading seconds field and the ? placeholder for DOM, which Quartz requires when DOW is specified.

Example 7, POSIX 0 8 * * 7 (Sundays at 08:00): description "Runs at 08:00 on Sunday." Demonstrates the 7 = Sunday POSIX extension.

Where It Shows Up

• Linux / Unix crontab, the canonical place. Edit with crontab -e; each user has their own table; the system table is /etc/crontab. Schedules run in the server's local timezone unless you set CRON_TZ= per entry.
• systemd timers, .timer files use a stricter cron syntax plus OnCalendar= ISO-8601 forms. Run systemctl list-timers to see active jobs.
• Kubernetes CronJob, uses a slight extension of POSIX (you can suffix the year and use ?); the controller evaluates them in UTC unless you set timeZone on the spec.
• AWS EventBridge / CloudWatch Events, six-field cron with * meaning "any"; the timezone is always UTC.
• GitHub Actions schedule:, POSIX five-field cron, UTC only; the smallest interval is 5 minutes.
• Spring @Scheduled(cron = "..."), uses Quartz six-field syntax; runs in the JVM's default timezone unless you set zone = "UTC" on the annotation.
• Java org.quartz.CronExpression, six-field with ? mandatory in DOM/DOW, also accepts L (last), W (weekday), # (nth weekday).
• Node.js node-cron, cron-parser, BullMQ repeatable jobs, all adopt POSIX 5-field with seconds-prefix option in some cases.

Common Mistakes

1. Confusing DOM and DOW syntax. 0 0 1 * 1 means "00:00 on the 1st OR every Monday," not "00:00 on the first Monday." If you need "first Monday of the month," Quartz gives you 0 0 0 ? * MON#1; classic Vixie cron has no built-in syntax for it.
2. Forgetting the timezone. A 0 9 * * * job fires at 09:00 in whatever zone the daemon runs in. Two servers in different zones will trigger the job at different wall-clock times, schedules diverge silently. Always pin the timezone either on the host (CRON_TZ, timeZone:) or by aligning systems on UTC.
3. Sub-minute expectations. Classic cron resolves only at the minute boundary. To fire every 30 seconds you must switch to Quartz (STAR/30 * * * * *) or run a sleep loop in the command. POSIX 5-field cron never supports sub-minute resolution.
4. Daylight-saving jumps. In America/New_York, a job scheduled at 0 2 * * * (02:30) either fires twice on the fall-back night (2:00 → 1:00 → 2:00) or never fires on the spring-forward night (2:00 → 3:00). Cron does not understand DST; you must guard against the duplicates / skips in the job itself, or use a scheduler that natively tracks wall-clock time (systemd, AWS EventBridge).
5. Forgetting that 7 = Sunday. Some tools (JavaScript's Date.getDay()) treat 0 as Sunday and 6 as Saturday; some treat 0 as Monday. POSIX lets you use either 0 or 7 for Sunday. This tool accepts both.
6. Step on a range applied to the wrong field. STAR/2 in the minute field gives "every 2 minutes"; STAR/2 in the hour field gives "at minute 0 every 2 hours." Same syntax, very different meaning, read left-to-right.
7. Empty fields and extra spaces. A trailing space or accidental second space silently splits or breaks a field. This tool shows you an explicit "5-field vs 6-field" indicator under the input so a missing field is obvious.
8. Assuming Quartz in a POSIX cron tool. Spring and Quartz expressions start with seconds. 0 9 * * * in Spring means "at second 0 of minute 9"; in Vixie cron it means "at minute 0 of hour 9". Pick the right dialect.
9. Not testing DST transitions. Schedule a job for 02:30 on a date in late October or late March and watch what happens, many "buggy" cron jobs are actually DST bugs the author never noticed.

Frequently Asked Questions

What is a cron expression? A cron expression is a compact string of five or six whitespace-separated fields that describes a recurring schedule. The classic 5-field POSIX layout is "minute hour day-of-month month day-of-week"; the Quartz 6-field layout adds a leading "seconds" field. Each field accepts values, lists (1,15), ranges (9-17) and steps (STAR/5).

Why is there a day-of-week AND a day-of-month? Because calendars have two ways to name a day: by its date in the month ("the 1st") and by its weekday ("Monday"). Most schedulers let you use either independently. When both are restricted, classic POSIX cron uses OR logic (fires if either matches), while Quartz requires one to be ? to disambiguate.

How do I schedule "every weekday at 08:00"? Use 0 8 * * 1-5. The 1-5 is a day-of-week range covering Monday through Friday; 0 8 fixes the time at 08:00 sharp. To skip weekends entirely (most business apps), this is the canonical form. Note the timezone: if the server is in UTC and your business is in Europe/London, you'll need CRON_TZ=Europe/London before the line in /etc/crontab, or the JVM equivalent.

How do I run a job every 30 seconds? Classic cron cannot, it ticks once per minute. Switch to a six-field expression with STAR/30 * * * * * (fires at second 0 and second 30 of every minute). Quartz, Spring, and many Node.js libraries accept this. For sub-second granularity, look at systemd timer OnUnitActiveSec or a tail-based processor.

What happens to cron at a DST boundary? Vixie cron, k8s CronJob and most cloud schedulers have no DST awareness. A 0 2 * * * job will fire twice during a 1-hour fallback (when the wall clock goes 1:59 → 1:00 again), and will not fire at all during a 1-hour spring-forward (when the wall clock jumps 2:00 → 3:00). systemd and CRON_TZ= (where supported) do track wall-clock time correctly. Always test DST crossings or guard in the job itself.

What is the difference between POSIX cron and Quartz cron? POSIX cron (Linux, k8s, GitHub Actions, EventBridge) uses 5 fields and resolves only at minute boundaries. Quartz cron (Java's org.quartz.CronExpression, Spring @Scheduled, Node.js BullMQ) uses 6 fields with a leading seconds field, accepts extra characters like L (last day), W (nearest weekday), # (nth weekday) and ? (no-spec in DOM/DOW), and supports sub-minute schedules. This tool accepts both: 5 whitespace-separated fields = POSIX, 6 = Quartz.

What does STAR/5 mean? STAR/5 in a minute field means "every 5 minutes starting from 0", fires at minute 0, 5, 10, 15, …, 55 of every hour. The slash operator can also be written over a range: 0-30/10 means "at minute 0, 10, 20, 30." Step sizes must be a positive integer; an empty base (/5) is invalid.

What does @daily mean? @daily (also @midnight) is a cron alias that expands to 0 0 * * *, 00:00 every day in the scheduler's timezone. Other common aliases: @yearly (or @annually) = 0 0 1 1 * (midnight on 1 January), @monthly = 0 0 1 * *, @weekly = 0 0 * * 0, @hourly = 0 * * * *. @reboot runs once at startup and cannot be expanded into a fire schedule because its timing depends on when the daemon starts.

Which timezone does cron use? The host's system timezone, by default. Linux /etc/timezone or timedatectl shows the current zone. In /etc/crontab you can prefix a single line with CRON_TZ=Europe/London to override. Systemd timers honour the [Timer] section's Persistent=true and the system zone. K8s CronJob evaluates in UTC unless you set spec.timeZone (requires Kubernetes 1.27+). AWS EventBridge is UTC-only.

Can I run a job on the last day of every month? Yes, but you need a sixth field with L. In Quartz: 0 0 L * ? means "midnight on the last day of every month." Classic Vixie cron has no built-in "last day" syntax; the common workaround is to schedule on days 28 to 31 and let the job check whether date +%d -eq $(cal | awk 'NR==6{print $NF}') (or similar) before doing real work.

Will cron drift if the server clock skews? Yes. Cron uses the system clock; if NTP isn't running and the clock drifts, cron fires at the drifted time. Run ntpdate or chrony and verify with timedatectl. Cloud schedulers (EventBridge, GitHub Actions) are immune because they use the provider's clock.

Can I use this tool for professional or commercial purposes? Yes. The calculator uses the same POSIX and Quartz semantics implemented by every mainstream scheduler (Linux cron, systemd, k8s CronJob, Spring @Scheduled, AWS EventBridge, GitHub Actions, BullMQ, node-cron). The output is suitable for production scheduling, on-call runbooks, and CI configuration. For high-stakes batch work, sanity-check against a test daemon before deploy.

How often are the underlying cron semantics updated? Never. The POSIX 5-field format has been stable since the 1980s (Vixie cron, 1987). Quartz added L/W/#/? in the 2000s and the syntax has been stable since v1. When a new field or operator is added by an implementation, this tool is updated to reflect the de-facto standard, see the field-reference table inside the tool for the current operator set.

References

1. Vixie cron (Paul Vixie, 1987), the de-facto POSIX cron reference. See man 5 crontab and the original release notes at ftp.vix.com/pub/vixie/cron-3.0.pl1.
2. Open Group Base Specifications Issue 7, §8.3, the POSIX specification for cron, including the field ranges and OR-rule for DOM/DOW. https://pubs.opengroup.org/onlinepubs/9699919799/utilities/crontab.html
3. systemd.timer(5), the modern Linux scheduler with first-class timezone and monotonic/real-time modes. https://www.freedesktop.org/software/systemd/man/systemd.timer.html
4. Quartz Scheduler, CronExpression Tutorial, the authoritative reference for 6-field cron including L, W, #. https://www.quartz-scheduler.org/documentation/quartz-2.3.0/tutorials/crontrigger.html
5. AWS EventBridge, Schedule Expressions for Rules, six-field cron + rate expressions, UTC. https://docs.aws.amazon.com/eventbridge/latest/userguide/eb-scheduled-rule-pattern.html
6. Kubernetes CronJob (batch/v1), adds year field to POSIX and supports timeZone from 1.27+. https://kubernetes.io/docs/concepts/workloads/controllers/cron-jobs/
7. GitHub Actions, schedule event, POSIX 5-field cron in UTC, minimum 5-minute interval. https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#schedule
8. Spring Framework, @Scheduled cron syntax, the Quartz 6-field dialect. https://docs.spring.io/spring-framework/reference/integration/scheduling.html

Also try these free tools:

• Cron Expression Builder, pick a schedule from dropdowns and copy the cron string.
• Cron Generator, alternative cron expression builder.
• Unix Timestamp Converter, convert dates to Unix timestamps for log analysis.
• Meeting Planner, schedule across multiple time zones.