How does the birthdate number of fifa work?

I used FIFAECP's data convertor, but the site is dead (what's going on with it?).

Is there any program the convert real dates to the fifa format?

How does the birthdate number of fifa work?
It is the number of days since the start of the Gregorian calendar (15 Oct 1582)

Is there any program the convert real dates to the fifa format?
The FIFA ECP calcdate.php is the only one that I know of. CM 07 does the conversion for you where appropriate, but I'm not aware of any stand-alone program (my EAFFAnalyser converts from the FIFA format to human-readable format but not the other way)