Javascript Date difference bug?

Edit: It's not a bug as Martin pointed out. I'm just crossing the daylight saving time, hence the 1h difference. I want to calculate the difference in days between "Mar 29 2010" and "Mar 09 2010" so i have the following code: ``` ((new Date(2010, 2, 29)).getTime() - (new Date(2010, 2, 8)).getTime()) / 86400000 ``` 86400000 is the number of milliseconds in a day and the difference between the dates is returned in milliseconds, so this should work. Only it doesn't quite. I get ``` 20.958333333333332 ``` It's the difference between those 2 dates that is wrong. It's supposed to be 1814400000 (21 days times 86400000 ), but it actually is 1810800000. Moreover if I change the difference to: ``` ((new Date(2010, 2, 28)).getTime() - (new Date(2010, 2, 7)).getTime()) / 86400000 ``` the same difference, only shifted one day back, i get normal results. This happens only if we try to get (x-y) where x is after March 29 2010 and y is before March 29 2010. I get this on Safari 4 and Firefox 3.6 on Mac, as well as IE 8 on windows 7. Haven't tried other browsers. Am I doing something wrong or is this a known bug ?