Cron Expression Parser

Parse and validate cron expressions. See human-readable descriptions and next scheduled run times.

Cron Expression

Format: minute hour day-of-month month day-of-week

Common Presets

What is Cron?

Cron is a time-based job scheduler found in Unix-like systems. It lets you run commands or scripts automatically at specified times - like setting an alarm clock for your computer.

Named after the Greek word "chronos" (time), cron has been scheduling tasks since the 1970s and remains the standard way to automate recurring jobs.

Common uses:

Database backups at midnight
Sending weekly email reports
Cleaning up old files
Syncing data between systems
Monitoring and alerts

Cron Expression Syntax

A cron expression has 5 (or 6) fields:

┌───────────── minute (0-59)
│ ┌───────────── hour (0-23)
│ │ ┌───────────── day of month (1-31)
│ │ │ ┌───────────── month (1-12)
│ │ │ │ ┌───────────── day of week (0-6, Sun=0)
│ │ │ │ │
* * * * *

Special characters:

Symbol	Meaning	Example
`*`	Any value	`* * * * *` = every minute
`,`	List	`1,15,30` = at 1, 15, and 30
`-`	Range	`1-5` = 1 through 5
`/`	Step	`*/15` = every 15

Common Cron Examples

🕐 Time-based:

Expression	Description
`0 * * * *`	Every hour (at minute 0)
`0 0 * * *`	Every day at midnight
`0 9 * * *`	Every day at 9:00 AM
`0 9 * * 1`	Every Monday at 9:00 AM
`0 0 1 * *`	First day of every month

⏱️ Interval-based:

Expression	Description
`/5 * * *`	Every 5 minutes
`/15 * * *`	Every 15 minutes
`0 /2 * *`	Every 2 hours
`0 0 /3 *`	Every 3 days

📅 Complex schedules:

Expression	Description
`0 9-17 * * 1-5`	Hourly, 9AM-5PM, Mon-Fri
`0 0 1,15 * *`	1st and 15th of month
`30 4 * * 0`	4:30 AM every Sunday
`0 22 * * 1-5`	10 PM on weekdays

Memory trick: Think "minute, hour, day, month, weekday" - from smallest to largest time unit (mostly).

Special Strings

Many cron implementations support these shortcuts:

String	Equivalent	Description
`@yearly`	`0 0 1 1 *`	Once a year (Jan 1)
`@monthly`	`0 0 1 * *`	First of the month
`@weekly`	`0 0 * * 0`	Sunday at midnight
`@daily`	`0 0 * * *`	Every day at midnight
`@hourly`	`0 * * * *`	Every hour
`@reboot`	-	On system startup

Not all cron implementations support these shortcuts! Standard crontab on Linux does, but cloud schedulers may not. Always test your expressions.

Common Cron Mistakes

❌ Mistake 1: Forgetting the "at" vs "every"

# Wrong: Runs every minute of hour 9
* 9 * * *

# Right: Runs once at 9:00 AM
0 9 * * *

❌ Mistake 2: Day of week confusion

# Sunday = 0 (or 7 in some systems)
# Monday = 1
# Saturday = 6

❌ Mistake 3: Overlapping conditions

# This runs on the 15th AND on Fridays (OR logic!)
0 0 15 * 5

# Not just "15th if it's a Friday"

❌ Mistake 4: Time zone issues

Cron uses the system's timezone
UTC is safer for production servers
Be careful during daylight saving transitions

Cron Variants

Standard Unix Cron (5 fields)

* * * * * command

Extended Cron (6 fields - with seconds)

* * * * * * command
│
└── seconds (0-59)

Used by: Quartz (Java), Spring, node-cron

AWS CloudWatch (6 fields - with year)

* * * * * *
          │
          └── year (optional)

Kubernetes CronJob Uses standard 5-field format, but runs containers instead of commands.

Best Practices

🔧 For Reliability:

Always log your cron job output
Add error handling and notifications
Use lock files to prevent overlapping runs
Set timeouts for long-running jobs

📝 For Maintenance:

# Good: Document your crontabs
# Backup database every night at 2 AM
0 2 * * * /scripts/backup.sh

# Bad: No comments, hard to understand later
0 2 * * * /scripts/backup.sh

⏰ For Performance:

Avoid running heavy jobs at "round" times (many jobs run at midnight!)
Stagger jobs: use 3 0 * * * instead of 0 0 * * *
Consider system load and resource contention

🔒 For Security:

Use absolute paths in scripts
Don't store credentials in crontab
Run with minimum required permissions