300b Begin counting down from 300 seconds (5 minutes).
5:00:00b Begin counting down from 18000 seconds (5 hours).
b Begin counting down from same place as last time.
300l Load countier to 300 seconds; don't start counting.
l Load countier to same place as last time; don't start counting.
p Pause, or Proceed from pause.
u Start counting up from zero.
s Record a Split time.
c Clear message buffer.
2n Play noise #2.
n Stop all currently-playing noises.

On android: Click on the small blue square to bring up the soft keypad. Use the numbers page not the letters page to enter numbers.

Background colors: green means running, yellow means paused, red means game over.

Free, libre, open-source, public domain, cross-platform HTML and javascript. Obviously no warranty. Tested on Firefox (Linux) and Chrome (Android). Probably works on other platforms. Try it online, then download it for offline use. You should download the files listed below. On any given platform, you only need half of the audio files, but it's hard to predict which half.