A Countdown Tracker That Handles DST, Timezones, and Calendar-Day Math Correctly

"Days between two dates" sounds trivial, but (to - from) / 86400000 gives wrong answers around DST transitions and can even be off by a day depending on local timezone vs UTC. The correct approach: convert both dates to UTC midnight, subtract, divide. That way a "day" is always 24 hours and the answer is in calendar days regardless of when the user's clocks shift.

Countdown timers are everywhere — wedding sites, exam apps, event pages. Most of them get date math subtly wrong. Twice a year, "10 days until the event" briefly shows as "9 days 23 hours" because DST moved the clock forward. The fix requires knowing that JavaScript Dates are wall-clock local time but arithmetic should be done in UTC.

🔗 Live demo: