How do I set the time zone of MySQL?

On one server, when I run:

mysql> select now();
+---------------------+
| now()               |
+---------------------+
| 2009-05-30 16:54:29 |
+---------------------+
1 row in set (0.00 sec)

On another server:

mysql> select now();
+---------------------+
| now()               |
+---------------------+
| 2009-05-30 20:01:43 |
+---------------------+
1 row in set (0.00 sec)