Build a cron expression

Why a visual cron builder?

A cron expression fits in five fields, yet remains one of the biggest sources of bugs in production. How many times has a developer written 0 0 * * 1 thinking "every Monday", forgetting that 1 can mean Sunday depending on the implementation? How many times has someone confused */15 and 15? The cron builder answers that need: visually compose a crontab expression, from common presets or individual fields, and get the final expression ready to paste into a Linux crontab, a Kubernetes CronJob or a GitHub Actions schedule.

The tool is the inverse of our cron decoder: where the converter takes an expression and unrolls the next executions, the builder takes human choices and produces the matching cron expression.

How it works

The graphical interface offers two complementary composition modes:

• Preset mode: a frequency selector (every minute, every hour, every day, every week, every month, every year, custom) that immediately generates a valid cron expression.
• Fields mode: five editable fields (minute, hour, day of month, month, day of the week) where you freely enter *, an integer, a range 1-5, a list 1,15,30 or a step */15.

The builder updates the expression in real time and shows a short human reading ("every day at 3am", "every 15 minutes") that acts as a safety net: if the sentence does not match your intent, the expression is probably wrong.

The five cron fields, in plain English

A standard cron expression contains five fields separated by spaces:

minute  hour  day-of-month  month  day-of-week
*       *     *             *      *

• minute (0 to 59);
• hour (0 to 23);
• day of month (1 to 31);
• month (1 to 12 or abbreviations Jan, Feb, etc.);
• day of week (0 to 7, with 0 and 7 both meaning Sunday in most implementations).

Each field accepts several forms: * (any value), a specific integer (5), a list (1,3,5), a range (9-17), a step (*/15 every 15 values), or a combination (0-30/5).

Typical use cases

• Developer learning cron: visualise how each field interacts, without risking a wrong expression in production.
• Busy sysadmin: generate a schedule in a few seconds for a Linux cron, a Kubernetes CronJob or an Ansible job without rereading the man page.
• Developer without POSIX experience: produce a valid crontab expression for Symfony Scheduler, Laravel Scheduler or node-cron without knowing the legacy syntax.
• DevOps in review: quickly prototype an expression to propose in a pull request, then validate it with the cron decoder.
• Legacy migration: reformulate a historical cron expression through the builder to make sure the semantics is correct before pushing it elsewhere.

Common presets

Selecting a preset overwrites the individual fields and produces the matching expression:

• Every minute: * * * * *. Avoid in production unless really needed.
• Every hour: 0 * * * *. On every hour start.
• Every day: 0 0 * * *. At midnight.
• Every week: 0 0 * * 0. Every Sunday at midnight.
• Every month: 0 0 1 * *. The 1st of the month at midnight.
• Every year: 0 0 1 1 *. January 1st at midnight.

Common patterns to know

Composing a custom expression

For more specific cases, the assistant exposes the five fields as free-text input. A few useful examples:

• Every day at 3am: minute=0, hour=3, other fields *.
• Every 15 minutes: minute=*/15, other fields *.
• Monday to Friday at 9am: minute=0, hour=9, day-of-week=1-5.
• 1st and 15th of the month at noon: minute=0, hour=12, day-of-month=1,15.
• Every 5 minutes between 9am and 5pm: minute=*/5, hour=9-17.

How to use the cron builder

1. Pick a preset (every minute, every hour, every day, every week, every month, every year) that matches the basis of your need.
2. Refine if needed by directly editing the five fields (minute, hour, day of month, month, day of week).
3. Read the human sentence generated by the tool to confirm the intent.
4. Copy the produced cron expression with the copy button.
5. Validate the schedule with our cron decoder which computes the next real execution dates.
6. Paste the expression into your Linux crontab, your Kubernetes CronJob, your Symfony Scheduler or your GitHub Actions workflow.

Classic pitfalls

• Day of month and day of week combined: if neither is *, behaviour differs across implementations. Linux/Vixie cron applies an OR; modern libraries sometimes behave differently. Avoid.
• Day 1 = Monday or Sunday?: depending on the country and convention. Most Unix crons consider Sunday=0 and Saturday=6. Check your scheduler.
• Time zone: a Unix cron uses the server's time zone by default. Application schedulers (Symfony Scheduler, Quartz) often allow a dedicated zone to be set.
• Missing minute: * * * * * runs the job every minute, which is rarely what you want. Remember to set a precise trigger (0) for hourly tasks.

Human reading and output

Alongside the cron expression, the tool offers a short human reading ("every day at midnight", "every 5 minutes"). It is a safety net: if the sentence does not match your intent, the expression is probably wrong. For a complex case, then use our cron decoder to compute the actual next execution dates.

Frequently asked questions

Example request

curl -X POST https://cdrn.fr/api/v1/tools/cron-builder/execute \
  -H "Content-Type: application/json" \
  -d '{"preset":"none","minute":"*","hour":"*","day_of_month":"*","month":"*","day_of_week":"*"}'

Input schema

Endpoints

• GET https://cdrn.fr/api/v1/tools - lists every available tool
• GET https://cdrn.fr/api/v1/tools/cron-builder - returns the schema for this tool
• POST https://cdrn.fr/api/v1/tools/cron-builder/execute - runs this tool with a JSON payload