This parses, validates, and explains cron expressions using the croniter library under MIT license. You can feed it a cron string and get a plain English explanation, calculate the next N run times, or validate syntax before deploying. It also claims to generate cron expressions from natural language descriptions like "every day at 9am," which is handy if you always forget the field order. The commands are straightforward Python scripts. Honestly, this is the kind of tool you don't think about until you're staring at "*/15 * * * *" at 2am wondering if it means what you think it means.
npx skills add https://github.com/aidotnet/moyucode --skill cron-scheduler