Online Timer

How the countdown keeps time

When you press Start the timer works out an end moment: the current clock time plus the duration you entered. Four times a second it subtracts the current time from that end moment and shows the result, rounded up to the next whole second, so the display reads 00:01 until the end moment has actually arrived. Counting this way means a slow or busy device cannot make the timer run long; it always finishes at the planned clock time while the page is visible.

Pause stores the remaining time to the millisecond, and Resume creates a new end moment from that exact figure, so no fraction of a second is lost or gained however many times you stop and continue; the finish moves later by precisely the length of each pause.

The alarm tone

The alarm is generated on the spot with the Web Audio API rather than played from a sound file: a sine wave at 880 Hz (the A above concert pitch) that fades in over 20 milliseconds and dies away over about 1.2 seconds. It plays once. There is no repeating ring and no stop button for the sound, so you cannot miss it by leaving it ringing, but you can miss it by being out of the room.

If the browser refuses to create audio, for example because the tab is muted or the device has no output, the timer still finishes visually; the failure is silent by design.

Background tabs, locked phones and late alarms

The check that fires the alarm runs on a JavaScript timer, and browsers throttle those in tabs you cannot see. Chrome, for instance, allows one wake-up per second in a hidden tab and, after about five minutes hidden, only one per minute. The remaining time shown is still right when you switch back, because it is recalculated from the end moment, but the tone itself can sound up to a minute late. On a phone, locking the screen or switching apps usually suspends the page entirely, and the alarm plays only when you reopen it.

For anything where a late alarm matters, keep the tab in the foreground or in its own window, or use the clock app built into your phone alongside this page.

Common timings and what to enter

If something does not work

• Start does nothing: the three fields add up to zero. A blank field counts as zero, so check that at least one of them has a number.
• No sound at the end: check the device volume, whether the tab is muted (right-click the tab in desktop browsers) and whether a site setting blocks sound.
• The display shows 05:00 before you type anything: that is the default of 0 hours, 5 minutes, 0 seconds, and Start uses it as is.
• You want to change the duration mid-count: press Reset, which returns you to the input fields with your previous numbers still in them.