John Resig does some extensive tests after noticing some odd behavior in JS speedtests and notes his findings:
Effectively these browsers (IE, Opera, Safari, and WebKit Nightly – all on XP/Vista) are only updating their internal getTime representations every 15 milliseconds. This means that if you attempt to query for an updated time it’ll always be rounded down to the last time the timer was updated” and “Any test that takes less than 15ms will always round down to 0ms in these browsers. It becomes impossible to determine how much time the tests are taking with consistently zeroed out results.