mysql> SELECT id, post_date FROM wp_posts2;
+-----+---------------------+
| id  | post_date           |
+-----+---------------------+
| 234 | 2012-01-20 13:16:39 |
| 235 | 2012-01-21 10:52:16 |
| 236 | 2012-01-22 14:07:31 |
| 240 | 2012-01-22 14:20:22 |
| 243 | 2012-01-23 23:01:27 |
| 245 | 2012-01-24 13:31:12 |
| 252 | 2012-01-24 23:04:01 |
| 256 | 2012-01-25 10:30:57 |
+-----+---------------------+
8 rows in set (0.00 sec)

mysql> SELECT id, post_date FROM wp_posts2 WHERE DATE(post_date)='2012-01-22';
+-----+---------------------+
| id  | post_date           |
+-----+---------------------+
| 236 | 2012-01-22 14:07:31 |
| 240 | 2012-01-22 14:20:22 |
+-----+---------------------+
2 rows in set (0.00 sec)

mysql> SELECT id, post_date FROM wp_posts2 WHERE DATE(post_date) BETWEEN '2012-01-22' AND '2012-01-23';
+-----+---------------------+
| id  | post_date           |
+-----+---------------------+
| 236 | 2012-01-22 14:07:31 |
| 240 | 2012-01-22 14:20:22 |
+-----+---------------------+
2 rows in set (0.00 sec)