Countdown Timer

What it is for

Anything with a fixed length and a room full of people: a talk, a test, a quiz round, a stretch of practice, tea. The numbers fill the screen, so a timer on a spare monitor or a projector is readable from the back, and full screen removes everything else.

Up to 600 minutes — ten hours — which is well past anything a browser tab should be trusted with, for reasons two sections down.

The clock, not the ticks

The countdown is measured against your computer's clock rather than by counting upward in the background. Browsers slow down and even freeze hidden tabs, which makes a naive timer fall behind; this one asks the clock every time it repaints, so switching tabs for five minutes takes five minutes off, exactly as it should.

While it runs, the page asks your browser to keep the screen awake, and it lets go the moment you pause, hide the tab or leave. Not every browser allows that, and where yours does not the page says so rather than letting you find out when the screen goes black.

What a browser tab cannot promise

The time will be right when you come back, but the chime can arrive late, or not until you return: a background tab may be throttled or frozen, and a sleeping phone may suspend it altogether. For anything that matters — an oven, a medicine, a train — use your device's own alarm, which the operating system guarantees.

For work-and-break intervals rather than one duration, use the Pomodoro Timer; for counting up instead of down, the Stopwatch; and for a specific time of day rather than a duration, the Alarm Timer.