We now correct for incorrect system clocks by doing the differential from page load with a server offset in account. Basically we just record the difference between your time and server time when we load the page, then add it to the differential when doing relative date calculations.
Bottom line: when it says "2 minutes ago" and updates past that, it should be accurate.
Let me just say now, if you change your system time after loading a page, yes your times will be off - no we're not going to cover that. If you do it and file a bug report on it we'll figure out appropriate retribution, and it will involve salad forks and eyeballs.