cron-doctor

Overview

Cron is deceptively error-prone. The failure mode is silent — a syntactically valid expression that simply never fires, or fires far more often than intended. 0 0 30 2 * parses cleanly and then sits dead forever (February has no 30th). 0 0 1,15 * 1 looks like "1st and 15th if Monday" but actually means "1st, 15th, OR every Monday" — ~6 fires/month instead of ~2.

This skill teaches an agent to catch those before they reach production. It comes with a zero-dependency validation engine (scripts/cron-engine.js, no install needed) that parses, describes, deep-validates, and computes next fire times.

When to Use This Skill

• Use when a user writes, edits, reviews, or deploys a cron expression — in a crontab, a Kubernetes CronJob, a GitHub Actions schedule, an Airflow DAG, a Celery beat schedule, a systemd timer, or any scheduled task.
• Use when debugging a job that "didn't fire" or "fired at the wrong time."
• Use when a user asks "what does this cron expression mean?" or "when will this run next?" or "how often does this run per year?"
• Use when reviewing a CI/CD pipeline or infrastructure config that contains a schedule field.
• Use when a user pastes a 5-field cron expression and asks for a sanity check.

How It Works

Step 1: Parse the expression

Split on whitespace into 5 fields: minute, hour, day-of-month, month, day-of-week. Confirm valid ranges:

Step 2: Describe it in plain English

State what the user thinks it does vs. what it actually does. Be explicit about OR-vs-AND semantics for day-of-month + day-of-week (see death-trap #2).

Step 3: Run the trap checklist

Check the five death-traps below and flag any that apply.

Step 4: Calculate next runs and annual fire count

Compute the next 5 fire times as concrete dates so the user can verify the schedule behaves as expected. Estimate annual fire count — a schedule that fires 365×/year vs. 12×/year is a ~30× cost and load difference.

The Five Cron Death-Traps

These are the bugs that pass crontab -l validation but break in production.

1. Impossible dates — the "never fires" bug

0 0 30 2 *

Valid syntax. Never fires. February has no 30th. This schedule is a dead job that silently sits forever. The same applies to day 31 in any 30-day month: 0 0 31 4 *, 0 0 31 6 *, 0 0 31 9 *, 0 0 31 11 *.

Fix: use 0 0 28-31 * * and check for end-of-month in the script, or use L (last day) syntax if your scheduler supports it.

2. OR-semantics — the "fires too often" bug

0 0 1,15 * 1

Does NOT mean "midnight on the 1st and 15th if it's Monday." Does mean "midnight on the 1st, the 15th, OR every Monday." That's ~6 fires/month instead of ~2.

This is the single most misunderstood cron rule. When both day-of-month AND day-of-week are restricted (neither is *), cron uses OR logic, not AND.

Fix: if you need "1st and 15th only if Monday," run daily and check in the script:

0 0 * * 1 [ "$(date +%d)" = "01" -o "$(date +%d)" = "15" ] && your-command

3. Midnight spike — the "everything at once" bug

0 0 * * *

Every job scheduled at 0 0 competes for resources at exactly 00:00. Database backups, log rotations, cert renewals, report generation — all fire simultaneously. This causes load spikes, connection-pool exhaustion, and cascading timeouts.

Fix: stagger jobs across the hour. Use 17 2 * * * or 43 3 * * * instead of 0 0. Jitter is your friend.

4. Uneven steps — the "drift" bug

*/7 * * * *

Does NOT mean "every 7 minutes evenly." It means "every 7 minutes starting at 0, then resets at 60." So: 0, 7, 14, 21, 28, 35, 42, 49, 56 — then 0 again (a 4-minute gap). The intervals drift: 7,7,7,7,7,7,7,7,4.

Fix: 60 is not divisible by 7. Use step values that divide 60 evenly: */5, */10, */15, */20, */30. If you truly need every-7-minutes, use a loop with sleep 420.

5. Leap-year February 29 — the "annual surprise"

0 0 29 2 *

Fires only on leap years — February 29, 2024 / 2028 / 2032… If someone writes this expecting "end of February," they'll be confused for 3 out of every 4 years.

Fix: use 0 0 28 2 * and handle the 29th case in the script if needed.

Using the validation script

This skill ships a zero-dependency engine at scripts/cron-engine.js (Node.js, no npm install needed). You can use it programmatically or from the CLI:

// Programmatic — Node.js, zero dependencies
const { describe, validate, nextRuns, formatNextRuns } = require('./scripts/cron-engine.js');

// Parse + describe -> returns { text, error, parsed }
const d = describe('0 0 30 2 *');
console.log(d.text);   // "At 00:00, on day-of-month 30 in in FEB"

// Deep validation -> catches the traps
const result = validate('0 0 30 2 *');
console.log(result.valid);              // true (syntax is valid)
console.log(result.observations);       // includes the "never fires" insight
console.log(result.suggestions);        // e.g. "Midnight is a common spike..."

// Next 5 fire times -> returns Date[]
const runs = nextRuns('0 9 * * 1-5', new Date(), 5);
console.log(formatNextRuns(runs, new Date())); // [{ date, relative, formatted }, ...]

# CLI (via the bundled wrapper)
node scripts/cli.js describe "*/5 * * * *"
node scripts/cli.js validate "0 0 30 2 *"
node scripts/cli.js next "0 9 * * 1-5" 5

Common cron presets

Best Practices

• ✅ Always provide the plain-English description AND run the trap checklist.
• ✅ Stagger midnight jobs to avoid the spike.
• ✅ Prefer step values that divide 60 evenly (*/5, */15, */30).
• ✅ Add a comment above every crontab line explaining intent.
• ✅ Set an explicit timezone (CRON_TZ) on schedulers that support it.
• ❌ Don't trust crontab -l validation — it only checks syntax, not semantics.
• ❌ Don't restrict both day-of-month and day-of-week without confirming OR-logic.
• ❌ Don't schedule everything at 0 0.

Common Pitfalls

• Problem: "My cron job isn't running." Solution: Check for an impossible date (trap #1) and confirm the daemon is running (service cron status / systemctl status crond). Verify the file ends with a newline and has correct ownership.

• Problem: "My job runs far more often than expected." Solution: You hit OR-semantics (trap #2). If both day-of-month and day-of-week are set, cron ORs them. Move one to * or guard in-script.

• Problem: "Intervals are uneven — sometimes 7 min, sometimes 4." Solution: Step value doesn't divide 60 evenly (trap #4). Use a divisor of 60.

• Problem: "My job works locally but not in the cluster." Solution: Timezone mismatch. Kubernetes CronJob and GitHub Actions default to UTC. Confirm timeZone / TZ is set as intended.

Limitations

• This skill targets standard 5-field cron as implemented by Vixie cron, systemd timers, Kubernetes CronJob, GitHub Actions schedule, and most libraries. It does not validate Quartz 6/7-field expressions with seconds/years, nor non-standard @reboot / L / # extensions without a note.
• Estimated annual fire counts assume a non-leap reference year; February 29 schedules (trap #5) are flagged explicitly.
• This skill does not replace environment-specific validation, testing, or expert review. Stop and ask for clarification if required inputs, permissions, or safety boundaries are missing.

Related Skills

• docker-expert — when the cron job runs inside a container and the issue is the container/entrypoint rather than the schedule.
• kubernetes-deployment — when validating a CronJob manifest's spec.schedule field alongside the broader resource config.

Security & Safety Notes

This skill is read-only and risk: safe. The validation script performs no file writes, network calls, or mutations — it only parses and computes. It is safe to run against any cron expression without preconditions.