This is a variation of the Hack and Tell Timer using seconds instead of minutes. I use it for timing iterations in Agile games like the Lean Lego Game.
Andrew Gwozdziewycz originally made this for Hack and Tell but used minutes instead of seconds.
Just dump all the files in a directory and load up index.html in your browser. If you append #45 to the url, you'll get a 45 second timer. It defaults to 30.
Double click on the time resets it. Single click pauses/unpauses.