mysql> select datediff('2006-10-24', '2006-10-24') as v1, datediff('2006-10-24', '2006-10-25') as v2, datediff('2006-10-24', '2006-10-26') as v3;
+------+------+------+
| v1   | v2   | v3   |
+------+------+------+
|    0 |   -1 |   -2 |
+------+------+------+
1 row in set (0.00 sec)

mysql> select timestampdiff(day, '2006-10-24', '2006-10-24') as v1, timestampdiff(day, '2006-10-24', '2006-10-25') as v2, timestampdiff(day, cast(now() as date), '2006-10-26') as v3;
+------+------+------+
| v1   | v2   | v3   |
+------+------+------+
|    0 |    1 |    2 |
+------+------+------+
1 row in set (0.00 sec)