By default, the timer operates as follows: it counts down from five minutes to zero minutes, updating every ten seconds. When one minute remains, the background turns yellow and the timer is updated every five seconds. At 30 seconds remaining, the background turns red and updates are continuous. Finally, when time runs out, the counter starts blinking.
Nearly all aspects of the counter can be configured by changing entries in the following form. If you want to customize it a lot, you can bookmark the counter page (after hitting "Start the Timer") and your bookmark will remember all the options you chose.
(P.S. Special thanks to Charles Peresso of Malta, who suggested the pause, reset, and "add time" features.)