Don't many mobile phones include stopwatch applications? I know my Sony Ericsson from 2004 had one.
Having neither an iPhone nor an Android phone, I do not know whether they include stopwatches, but I would be more likely to use a native application version than a mobile website.
I clicked on the tour and noticed the graphing feature - this should be on the front page. To me, seeing progress / time is more useful than a stopwatch app by itself.