How the browser alarm works
You enter the hour and minute at which you want the alarm to fire, using either 12-hour (AM/PM) or 24-hour format. The alarm reads the current local time from your device's system clock, calculates the difference between now and the target time, and begins counting down in real time. The page updates every second using the browser's built-in timer API.
When the countdown reaches zero, the alarm triggers a sound (using the Web Audio API if your browser supports it, otherwise a pre-loaded audio file) and flashes a visual banner on screen. Most browsers require a one-time user interaction — clicking a button on the page — before they allow audio playback. This is a browser security policy, not a limitation of the alarm clock itself.
Countdown (s) = Target time (Unix s) − Current time (Unix s)
Recalculated every second; fires alert when value reaches zero.