Online Countdown Timer

A countdown in your browser

The timer uses the device clock and an absolute end time, refreshing about every 100 ms. It calculates the remaining time from that end time instead of subtracting a fixed amount at every callback. A delayed browser callback can still delay what you see, and a system clock change can alter the remaining duration.

Display and controls

The total remaining duration is rounded upward to whole seconds, then split into hours, minutes and seconds. For example, 59.999 seconds displays 00:01:00, and 1 hour 2 minutes 3 seconds minus 30 seconds displays 01:01:33. Hours can use three digits for long durations. The initial display is zero until you start or reset.

Pause refreshes the display and saves the current remaining time. Start resumes that remainder; editing the fields alone does not replace it. Reset stops the timer, rereads the current fields and displays that duration. Invalid values such as 60 minutes are rejected when the duration is read. For 90 minutes, enter 1 hour and 30 minutes.

Limits

The countdown and its state stay in the page. Reloading or closing the page loses the running timer. There is no sound, operating-system notification, repeating timer or saved preset. Browser throttling, suspension and device sleep can delay the completion message. This is a convenience timer rather than an alarm with guaranteed timing.