Cross-browser Javascript Datediff or Date difference function

In javascript you don't get any straight cut way to calculate the date or time difference between two dates. But in our lives most of times we need to calculate the time difference between two dates. Javascript gives us a built in function named getTime() which returns the number of milliseconds since midnight of January 1, 1970. Don't be confused since the base is 1970 for all so that we can