Cron Expression Builder — POSIX Cron, Quartz & GitHub Actions Technical Guide
Build, validate, and explain cron schedule expressions for POSIX cron, Quartz Scheduler, Kubernetes CronJobs, and GitHub Actions.
01. Cron Expression Format: POSIX 5-Field & Quartz 6/7-Field
POSIX cron uses a 5-field time expression: minute (0-59), hour (0-23), day-of-month (1-31), month (1-12 or JAN-DEC), day-of-week (0-7, where 0 and 7 both represent Sunday). Special values: * (any), , (list separator), - (range), / (step). Example: */15 * * * * runs every 15 minutes.
Quartz Scheduler (Java Spring applications) extends to 6-7 fields adding seconds at start and optional year at end. Quartz also supports L (last), W (nearest weekday), # (Nth weekday), and ? (no specific value) for more precise scheduling.
GitHub Actions and Kubernetes CronJobs use 5-field POSIX format with UTC. GitHub Actions has minimum ~15 minute resolution due to runner queue delays. Kubernetes CronJob timezone is configurable in .spec.timeZone (Kubernetes 1.27+).
- POSIX cron: 5 fields — minute hour dom month dow.
- Quartz: 6-7 fields — seconds minutes hours dom month dow [year].
- GitHub Actions schedule: 5-field UTC, effective minimum ~15 min resolution.
- Kubernetes CronJob: 5-field POSIX, timezone in .spec.timeZone (K8s 1.27+).
- Step values: */5 in minute = every 5 min; 0-30/5 = every 5 min from 0 to 30.
02. Common Cron Pitfalls & DST Edge Cases
Day-of-month and day-of-week OR semantics: In POSIX cron, if both DOM and DOW are non-wildcard, the job runs when EITHER is true (OR semantics, not AND). Quartz uses ? to explicitly mark "not applicable" for one field.
Daylight saving transitions: when clocks spring forward, jobs between 2:00-3:00 AM are skipped. When clocks fall back, jobs in that window run twice. Always use UTC for production cron to avoid DST edge cases.
- DOM/DOW OR semantics: both non-* in POSIX means either condition triggers.
- Quartz ? field: marks one of DOM/DOW as "not applicable".
- DST safety: run production cron in UTC — avoids spring-forward skips and fall-back doubles.
- Month numbers: POSIX 1-12; JAN-DEC names also accepted for clarity.
Frequently Asked Questions (FAQ)
Difference between POSIX cron and Quartz cron?
POSIX uses 5 fields (minute hour dom month dow). Quartz adds seconds field at start and optional year at end (6-7 fields), plus L W # ? special characters for more precise scheduling.
Why does GitHub Actions cron not run every minute?
GitHub Actions has scheduler delays up to 15+ minutes due to runner queue capacity. Minimum reliable resolution is approximately 15 minutes.
Is my confidential data uploaded to external servers when using Cron Expression Builder?
No. Core processing in Cron Expression Builder executes in volatile browser memory without server uploads. Inputs to local tools are not logged on remote servers; optional cloud-assisted features require user consent.
Can I use Cron Expression Builder offline without an active internet connection?
Offline use requires the page and its tool assets to finish caching first. Core transformations can then run locally; optional AI and contact delivery require internet.
Is Cron Expression Builder free to use with any rate limits or paywalls?
Core tools are free to use without an account. Device memory and tool-specific safety limits apply. Optional AI and online contact services have usage limits.