I think a for loop is indeed better that a while loop, as you do the definition of the counter variable and the incrementation in one line :)

Really a clever solution! :)

No one dose!

Great solution, runeh.
You can also ignore the Math.abs() function as in the match[1] and match[2] is just the "remembered" part of the expression and this is the number before the optional "." without the "-".

Hmm, I do not understand how to start this.
...

Why is there the "||9"?

Edit: Ok, now I know. It's when digits of the birtdate calculate to "9".

Awesome. I was not aware that you can have the sum of the digits by doing modulo 9 operation.

Anyone knows why is that?

I found a nice way to convert a float into an int:

var intVal = floatVal | 0;

You can find it here: http://stackoverflow.com/questions/596467/how-do-i-convert-a-float-to-an-int-in-javascript

Also:

What happens when startDate is in 1969?

I tried "endDate.getTime()-startDate.getTime()/1000 | 0 and it worked :)