LP:738096 - DATEDIFF with a 0000-00-00 argument produces different result in 5.1-micro

The following expression:

SELECT DATEDIFF( '2004-08-07' , DATE ( '0000-00-00 00:00:00'));

returns NULL in 5.1-micro and 732165 in mysql-5.1

In this case it seems safer and backward compatible to allow people to use the 0000-00-00 date in datediff calculations. It is plausible that such an expression can occur in a real life query.