I finally understood cron expressions by building an explainer for them

For years I copied cron expressions off Stack Overflow, pasted them into a config file, crossed my fingers, and moved on. 0 9 * * 1-5 ? Sure, that "looks like weekday morning." */15 * * * * ? "Every 15 minutes, probably." I never actually read them. So I did the thing that always cures this for me: I built a tool that parses a cron expression, explains it in plain English, and shows the next five times it will fire. No library. About 50 lines of real logic. Here's everything I learned. The five fields (and the order that trips everyone up) A standard cron expression is exactly five fields separated by spaces: ┌──────── minute 0 - 59 │ ┌────── hour 0 - 23 │ │ ┌──── day - of - month 1 - 31 │ │ │ ┌── month 1 - 12 │ │ │ │ ┌ day - of - week 0 - 6 ( 0 = Sunday ) * * * * * The order never changes, and the number-one beginner mistake is swapping the first two. Minute comes first. If you write 9 30 * * * thinking "9:30am," you actually get "minute 9, hour 30" — which is invalid, because hours only go to 23. Say it out loud every time: minute, hour, day-of-month, month, day-of-week. Each field answers one question: which values of this unit does the job run on? An * means "every value." Most real schedules pin down a couple of fields and leave the rest as * . Daily at 9am is 0 9 * * * — minute and hour fixed, everything else "every." Lists, ranges, and steps Beyond single numbers, each field understands three operators, and they combine: Comma makes a list: 1,15 in the day field means the 1st and the 15th. Hyphen makes an inclusive range: 1-5 in the day-of-week field means Monday through Friday. Slash makes a step, taking every n-th value: */15 in the minute field means 0, 15, 30, 45 . Steps can apply to a range too, so 0-30/10 means 0, 10, 20, 30 . That's the whole grammar. Number, list, range, step. Once you can expand a field into the concrete set of numbers it matches, you understand cron. Here's the expansion function, which is the heart of the parser: function expandFie