Countdown Timer

A countdown timer counts down from a duration you set and alerts you when it reaches zero. Building one accurately is harder than it looks. A naive implementation calls setInterval every 1000 milliseconds and decrements a counter, but JavaScript timers are not real-time and can fire late, so over a long countdown the timer drifts by seconds. This countdown timer avoids that by recording a fixed end timestamp the moment you press Start, then recomputing the remaining time on every animation frame from that anchor. The result finishes exactly on time regardless of CPU load or how aggressively the browser throttles the tab. When the count hits zero, the alarm is synthesized live with the Web Audio API as a clean 880 Hz sine tone, so there is no MP3 to download and no autoplay-policy delay before the first beep. If you grant permission, a system notification also fires, which means the alert still reaches you when the tab is buried behind dozens of others. The most common confusion is missing notifications: browsers require explicit permission, so the first time you press Start you may see a permission prompt, and if it was dismissed you can re-enable notifications from the site settings via the lock icon in the address bar. A second caveat is that background tabs are throttled, so the audio tone may be quieter or briefly delayed when the tab is hidden, though the notification and the zero event still register correctly. You can pause and resume without losing accuracy, and durations longer than 24 hours are accepted. Everything runs locally; your timer durations are never uploaded or logged.