Cron Doctor

cron-doctor

When to use this skill

Use this skill whenever a cron expression is involved:

• A user is writing or editing a crontab, Kubernetes CronJob, GitHub Actions schedule, Airflow DAG, or any scheduled task.
• A user is debugging a job that "didn't fire" or "fired at the wrong time."
• A user asks "what does this cron expression mean?" or "when will this run next?"
• A CI/CD pipeline includes a scheduled trigger that needs validation.

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. This skill catches those before they reach production.

The five cron death-traps

These are the bugs that pass crontab -l validation but break in prod:

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 will silently sit there forever. The same applies to 0 0 31 4 *, 0 0 31 6 *, 0 0 31 9 *, 0 0 31 11 * — any day 31 in a 30-day month.

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

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 (not *), 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 your jobs across the hour. Use 17 2 * * * or 43 3 * * * instead. 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 (3-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.

How to diagnose a cron expression

When a user provides a cron expression to check:

1. Parse it. Split on whitespace into 5 fields: minute, hour, day-of-month, month, day-of-week. Validate ranges: minute 0-59, hour 0-23, dom 1-31, month 1-12, dow 0-7 (0 and 7 both = Sunday).

2. Describe it in plain English. What the user thinks it does vs. what it actually does. Be explicit about OR-vs-AND semantics.

3. Run the trap checklist above. Flag any of the five death-traps.

4. Calculate next runs. Compute the next 5 fire times as concrete dates so the user can verify the schedule behaves as expected.

5. Estimate annual fire count. A schedule that fires 365 times/year vs. 12 times/year is a 30x cost difference. Make this visible.

Using the validation script

This skill includes a zero-dependency validation engine at scripts/cron-engine.js (638 lines, 69 tests). You can use it programmatically:

// Node.js — no install needed, zero dependencies
const { parse, describe, validate, nextRuns } = require('./scripts/cron-engine.js');

// Parse + describe
console.log(describe('0 0 30 2 *'));
// → "At 00:00 on day-of-month 30 in February"

// Deep validation — catches the traps
const result = validate('0 0 30 2 *');
// → { valid: true, warnings: ["February 30 never occurs — this schedule never fires"],
//     observations: [...], runsPerYear: 0 }

// Next 5 fire times
console.log(nextRuns('0 9 * * 1-5', 5, new Date()));
// → [Date, Date, Date, Date, Date]

You can also run it from the command line:

node scripts/cron-engine.js validate "0 0 30 2 *"
node scripts/cron-engine.js describe "*/5 * * * *"
node scripts/cron-engine.js next "0 9 * * 1-5" 5

Common cron presets

Always verify

When you help a user with cron, always provide the plain-English description AND run the trap checklist. Don't just say "looks good" — cron's whole danger is that invalid schedules look valid.