Developer Builds Dependency-Free Countdown Timer in Vanilla JS, Shares Lessons Learned

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A developer built a browser-based countdown timer using only vanilla JavaScript, HTML, and CSS, deliberately avoiding frameworks like React or third-party libraries. The goal was a lightweight, single-purpose tool that required no installation, account creation, or premium features — just a target date and a visual progress indicator. One key challenge was timezone handling: a user in Australia reported incorrect countdowns, which traced back to the app using local timezone offsets instead of storing dates as UTC timestamps. The fix involved converting date inputs to UTC millisecond timestamps to ensure consistent calculations across all regions. The developer also used AI tools like Claude and ChatGPT during development, and documented both the successes and limitations of that approach.

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