Here's a live demo.
This thing is pretty much self documenting.
Defaults to a 20 minute timer, double-click the time to change.
Alternately, and assuming you have python 2.x installed, run python py2serve.py from the command line and navigate to
http://localhost:8000/timer.html#<time for timer in seconds>, i.e. to get a 30
second timer go to http://localhost:8000/timer.html#30. If you're a python 3
purist run py3serve.py.