I finally understood cron expressions by building an explainer for them

Short summary

A developer's guide to understanding cron expressions by implementing a simple parser. Covers the five-field syntax (minute, hour, day-of-month, month, day-of-week), field operators (comma lists, ranges, steps), and surprising behavior like day-of-week/day-of-month OR matching. Includes practical JavaScript code and common pitfalls like the month off-by-one bug.

• •Cron syntax has five fields with three operators: commas for lists, hyphens for ranges, slashes for steps
• •The day-of-week and day-of-month fields use OR logic, not AND—either condition triggers the job
• •JavaScript's Date.getMonth() is off-by-one from cron's month field, a hidden bug in scheduling code

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