What's the best way to calculate date difference in Javascript

I doing a function in Javascript like the VisualBasic DateDiff.

You give two dates and the returning time interval (Seconds, Minutes, Days, etc...)

DateDiff(ByVal Interval As Microsoft.VisualBasic.DateInterval, _  ByVal Date1 As Date, ByVal Date2 As Date) as Long

So what's the best way to calculate the difference of Javascript Dates?