You Probably Don't Need a Timezone Library Anymore

Writing a Cron Parser and Multi-Timezone Next-Run Calculator in ~300 Lines of Vanilla JS A small, zero-dependency static web tool, and the two things that turned out to be actually interesting about building it. Every time I hand a cron expression to a teammate in another timezone, I do the same mental math. " 0 9 * * 1-5 in JST — that's 00:00 UTC, which for PT is… 16:00 the previous day during standard time, 17:00 during DST… hold on, is DST active right now?" crontab.guru is a wonderful tool, but it only shows one timezone at a time. So I built the thing I wanted: a static page that shows the next 10 runs of any cron expression across UTC / JST / PST / EST / CET in one table, side by side, DST-aware. 🔗 Live demo : https://sen.ltd/portfolio/cron-tz-viewer/ 📦 GitHub : https://github.com/sen-ltd/cron-tz-viewer The whole thing is plain HTML/CSS/JS with no build step and zero runtime dependencies. Total surface area: about 500 lines including tests. The two parts I want to write about are