Reading Cron Expressions: The 5-Field Format
Standard Unix cron uses 5 space-separated fields: minute hour day-of-month month day-of-week. Each field accepts numbers, ranges (1-5), lists (1,3,5), step values (*/15), or a wildcard (* for "any").
Common Cron Expression Examples Explained
* * * * * — Every minute, every hour, every day.
0 * * * * — Every hour, at the top of the hour (:00).
0 9 * * 1-5 — Weekdays (Monday–Friday) at 9:00 AM.
*/5 * * * * — Every 5 minutes.
0 0 1 * * — First day of every month at midnight.
30 2 * * 0 — Every Sunday at 2:30 AM (common for maintenance windows).
Crontab.guru vs This Tool — What's Different
crontab.guru is excellent for standard 5-field Unix cron (and is owned by Cronitor, a monitoring startup). It does not support AWS EventBridge cron format (which uses 6 fields including a year and requires ? for day-of-week/day-of-month), nor does it show computed next-run timestamps. This tool shows the next 5 actual datetime values when your job will fire.
Why Cron Jobs Often Fire at the Wrong Time
The most common mistake is timezone confusion. Cron runs in the server's local timezone by default — but most cloud servers (AWS, GCP, Azure) run in UTC. If you schedule a job for 0 9 * * * expecting 9 AM Eastern, it will fire at 9 AM UTC (4 AM or 5 AM Eastern depending on DST).
Frequently Asked Questions
What does "0 5 * * *" mean?
This runs once per day at 5:00 AM in the server's local timezone (usually UTC on cloud servers). Field breakdown: 0=minute, 5=hour, *=any day of month, *=any month, *=any day of week.
What does "*/15 * * * *" mean?
Every 15 minutes. The */15 in the minute field means "every 15 minutes starting from 0", so it fires at :00, :15, :30, and :45 of every hour.
How is AWS EventBridge cron different?
AWS EventBridge uses a 6-field format with an added year field: cron(minute hour day-of-month month day-of-week year). It also requires exactly one of day-of-month or day-of-week to be ? (meaning "not specified") since you cannot specify both.
Why is my cron job running at the wrong time?
Most likely a timezone issue. Check what timezone your server uses (usually UTC on cloud servers). If you want 9 AM EST, schedule "0 14 * * *" (UTC) during standard time or "0 13 * * *" during daylight saving time.
What is the difference between "0 9 * * 1-5" and "0 9 * * MON-FRI"?
They are equivalent. Both run at 9:00 AM Monday through Friday. The numeric form (1-5) and the name form (MON-FRI) both work in standard crontab. Note that 0=Sunday and 7=Sunday (both work), while 1=Monday through 6=Saturday.