Cron表現生成ツールおよび次の実行時刻計算機 – ギャンブルなしでマスターするCron

Cron syntax is one of those things developers look up every single time. Not because it’s hard — but because the five-field format is just opaque enough to cause doubt. Is the hour field before or after the minute? Does */5 mean every 5 minutes or starting at minute 5?

The “cron expression” tag on Stack Overflow alone has over 14,000 questions. That’s before you factor in the platform inconsistencies that multiply the confusion: Linux cron uses five fields, Kubernetes uses five fields with slightly different semantics, AWS EventBridge adds a sixth, and Quartz schedulers prepend a seconds field at the front.

Two tools cut through the noise: a visual cron expression generator that builds expressions from human-readable inputs, and a next-run calculator that shows you exactly when your job will fire — no guessing required.

Cron Syntax Refresher

The standard five-field cron format reads left to right: minute, hour, day of month, month, day of week.

* * * * *
│ │ │ │ └── Day of week (0–7, where 0 and 7 = Sunday)
│ │ │ └──── Month (1–12)
│ │ └────── Day of month (1–31)
│ └──────── Hour (0–23)
└────────── Minute (0–59)

A few common examples:

• 0 2 * * * — every day at 2:00 AM
• 0 9 * * 1 — every Monday at 9:00 AM
• */15 * * * * 特定の値なし（Quartz/AWSのみ）
• 0 0 1 * * — midnight on the first of every month

Simple in isolation. In practice, the ambiguity compounds: is 1 for the day-of-week Monday or Sunday? Does this platform support step values? This is where a generator earns its keep.

Tool 1: Cron Expression Generator

の Cron式ジェネレータ turns abstract syntax into a point-and-click interface. Set values for each component — minute, hour, day, month, weekday — and the tool outputs the expression. No memorizing field order, no second-guessing special characters.

Two real-world walkthroughs:

Daily database backup at 2 AM

Set Minute to 0, Hour to 2, leave Day of month, Month, and Day of week as *. Output: 0 2 * * *. This fires at exactly 02:00 every day.

Weekly report every Monday at 9 AM

Set Minute to 0, Hour to 9, Day of month to *, Month to *, Day of week to 1 (Monday). Output: 0 9 * * 1.

Where the generator really shines is with step values and ranges — */5 (every 5 minutes), 1-5 (Monday through Friday), 0,12 (midnight and noon). These are the combinations where the mental overhead of raw cron syntax becomes a liability, and a visual builder makes the logic explicit before it reaches production.

Tool 2: Cron Next Run Calculator

Getting the expression right is step one. Confirming it fires when you expect is step two — and it’s easy to skip.

の Cron表現：実用的な参考書と実際の例 1 takes any cron expression and shows the next 10 scheduled fire times, calculated from right now. Paste in your expression, get the output immediately.

A practical use case: you’ve written 0 9 * * 1-5 thinking it runs Monday through Friday at 9 AM. Paste it into the calculator and the next 10 run times surface immediately — making it obvious whether you have the right range. If you accidentally wrote 0 9 * * 0-5 (Sunday through Friday instead), the next-run output shows a Sunday firing and you catch the bug before deployment.

This is faster than deploying a job, waiting, and checking logs — especially for jobs scheduled to run once a week or once a month, where a misconfigured expression might go undetected for weeks.

Platform Differences: Linux, Kubernetes, GitHub Actions, and AWS

This is where cron gets genuinely messy. The same expression can mean different things — or be invalid — depending on which scheduler runs it.

Linux cron (5 fields)

The original. Five fields: minute hour day month weekday. Configured via crontab -e. No seconds support. Day-of-week: 0 と 7 both mean Sunday.

Kubernetes CronJobs (5 fields)

Kubernetes uses the same 5-field format for its schedule: field — functionally equivalent to Linux cron. Step values and ranges work the same way. It does ない add a seconds field despite what you may have read.

AWS EventBridge / CloudWatch Events (6 fields)

EventBridge uses a different 6-field format: minute hour day month weekday year. The year field is mandatory and cannot be omitted. It also uses ? の代わりに * when a field is not applicable — you must use ? for either day-of-month or day-of-week, since specifying both is ambiguous.

GitHub Actions (5 fields, UTC only)

GitHub Actions accepts standard 5-field cron syntax in the schedule: trigger. All times are UTC. The minimum schedule interval is 5 minutes, and scheduled jobs may fire later than scheduled during periods of high load on GitHub’s infrastructure.

Quartz / Spring Scheduler (6–7 fields)

Java-based schedulers like Quartz and Spring Scheduling add a 秒 field at the front: second minute hour day month weekday [year]. This means 0/5 * * * * ? fires every 5 seconds — not every 5 minutes. Easy to confuse if you’re switching between Java and Linux cron.

Common Cron Patterns

用途	表現	When it runs
15分ごと	*/15 * * * *	:00, :15, :30, :45 each hour
Daily at midnight	0 0 * * *	Every day, 00:00
Daily backup at 2 AM	0 2 * * *	Every day, 02:00
Weekdays at 9 AM	0 9 * * 1-5	Mon–Fri, 09:00
Weekly cleanup (Sunday midnight)	0 0 * * 0	Sundays, 00:00
First of month at noon	0 12 1 * *	1st of each month, 12:00
毎時	0 * * * *	Top of every hour
Twice daily	0 8,20 * * *	08:00 and 20:00 daily

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