Problema al restar fechas

[webness]

ENSAYA CON LA FUNCION DATEDIFF

Código MySQL:

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