cron-doctor

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,