mysql> SELECT DATEDIFF('1997-12-31 23:59:59','1997-12-30');
-> 1
mysql> SELECT DATEDIFF('1997-11-30 23:59:59','1997-12-31');
-> -31

DATEDIFF() was added in MySQL 4.1.1.

Et de manière générale, tout bon moteur aurait put te donner:

http://tinyurl.com/4xb3j