Online Cron Expression Parser: Visualize Scheduled Task Scheduling

What Is a Cron Expression

A Cron expression is a string format for configuring scheduled task execution, widely used in Linux crontab, CI/CD pipelines, Node.js task schedulers, and distributed job systems. Paste an expression into the Cron Expression Parser to see each field’s expanded legal values, a human-readable summary, and the next 5 runs in the browser’s local timezone — answers to “what does this string mean?” and “when does it fire on my clock?”

Next-run math follows Vixie day/weekday OR semantics and is labeled as browser-local time. It is not the target server calendar: cloud VMs are often UTC, and containers without TZ usually are too — convert the preview, or smoke-test with a low-frequency log probe on the box. To build from requirements, use the Cron generator.

A standard Cron expression consists of 5 time fields:

┌───────────── Minute (0-59)
│ ┌───────────── Hour (0-23)
│ │ ┌───────────── Day of Month (1-31)
│ │ │ ┌───────────── Month (1-12 or JAN-DEC)
│ │ │ │ ┌───────────── Day of Week (0-7 or SUN-SAT; 0 and 7 both mean Sunday)
│ │ │ │ │
* * * * *

Why You Need a Cron Expression Parser

1. Scheduled tasks: Server backups, data sync, report generation
2. CI/CD scheduling: Timed builds and deployments
3. Monitoring: Periodic health checks
4. Data processing: ETL jobs, log rotation
5. Understanding complex expressions: Expand fields and read the summary before you deploy

Cron Field Syntax

Special Characters

Common Cron Expressions

How to Use an Online Cron Parser

Using ToolVault's Cron Expression Parser:

1. Enter a five-field Cron expression (e.g., */15 * * * *, or 0 9 * * MON-FRI)
2. The tool expands each field into its legal value list (name aliases resolve to digits first)
3. Shows a human-readable summary assembled from those sets
4. Lists the next 5 runs (browser local timezone; OR when both day fields are restricted)
5. Wrong field count or truly out-of-range values (e.g. weekday 8) surface as clear errors; weekday 7 normalizes to Sunday 0
6. Start from built-in examples and tweak numbers or aliases while watching expansions and next runs

Cron in Code

Node.js

const cron = require('node-cron');

// Every 5 minutes
cron.schedule('*/5 * * * *', () => {
  console.log('Running health check');
});

// Daily at 2:00 AM
cron.schedule('0 2 * * *', () => {
  console.log('Running data backup');
});

Python

from crontab import CronTab

# Parse a Cron expression
cron = CronTab('* * * * *')
print(cron.description())  # "Every minute"

# Get next execution time
from datetime import datetime
next_run = cron.get_next(datetime)

Go

import "github.com/robfig/cron/v3"

c := cron.New()
// Every 5 minutes
c.AddFunc("*/5 * * * *", func() {
    fmt.Println("Running health check")
})
c.Start()

Important Notes

Timezone

Cron expressions default to the server's local timezone. In distributed systems, use UTC consistently:

# UTC time
0 2 * * *    # Daily at UTC 02:00

Sunday and name aliases

• This site’s parser: 0=Sunday … 6=Saturday; accepts 7 and normalizes it to 0 (Vixie-compatible)
• Months/weekdays accept three-letter English names: JAN–DEC, SUN–SAT (case-insensitive)
• Most docs: 1=Monday (confirm against your target implementation)

Minimum Granularity

Standard Cron's minimum granularity is minutes. For second-level scheduling, use extended Cron syntax (e.g., */5 * * * * * — 6 fields). Six-field strings return INVALID_CRON_FORMAT here — count fields for your scheduler dialect first.

The parsing model: five fields, then everything modern added

Classic cron: five fields with a subtle rule — when both day-of-month and day-of-week are restricted, they OR (not AND). The next-run table uses the same OR rule so misreads show up immediately. Modern extensions: seconds (Quartz / Spring), @daily/@hourly macros, k8s CronJob (five-field only — Quartz-with-seconds pastes fail confusingly). This site’s parser and generator bind the classic five-field dialect; count segments before cross-dialect paste.

Timezone: field expansion vs next runs are two layers

Field expansion does not use the browser timezone — it only turns */15 / MON-FRI into value sets. Next 5 runs use the browser-local clock and are labeled as such. What decides production “when it rings” is the daemon / container / CronJob clock: system cron uses server-local time; Docker often defaults to UTC; k8s can set timeZone; Quartz follows the JVM default. DST adds another trap: spring-forward hours never run, fall-back may run twice. Discipline: avoid the fuzzy 2–3am window; align the preview with the server timezone before go-live.

The five mistakes that make crons silently die

(1) Relative paths (cron’s cwd is not yours — cd explicitly); (2) missing environment (minimal PATH — source a profile or use absolute paths); (3) no output handling (mail nobody reads — redirect to a rotated log); (4) no lock (overlapping runs — flock); (5) no monitoring (dead for three weeks — heartbeat/deadman). Ops checklist: Linux crontab ops.

Implementer's note

describeCron collapses whitespace, requires exactly five fields, expands * / */n / a-b / a-b/n / a,b, and resolves month/weekday English names first; weekday 7 normalizes to 0. getNextCronRuns walks candidate times with month length and OR when both day fields are restricted. The description string remains an English phrase assembly — not a localized NL engine.

FAQ

What's the Relationship Between Cron Expressions and crontab?

A Cron expression is the string representation of a scheduling rule. crontab is the file/command in Linux for configuring scheduled tasks. You write Cron expressions into crontab to configure jobs.

Are */15 * * * * and 0,15,30,45 * * * * the Same?

They produce the same result — both execute every 15 minutes. */15 is shorthand step syntax.

How Do I Schedule Monday Through Friday?

Use 0 9 * * 1-5 or 0 9 * * MON-FRI (example: weekdays at 9am). In the day-of-week field, 1=Monday, 5=Friday, Sunday is 0 or 7 (equivalent here).

Which timezone do “next runs” use?

The preview uses the browser local timezone. If the target host is UTC, convert yourself, or smoke-test with a low-frequency date >> /tmp/cron-smoke.log job on the server.

Written and maintained by ToolVault. The homepage collects more browser-local tools.