How does one create an index on the date part of DATETIME field in MySql

How do I create an index on the date part of DATETIME field?

mysql> SHOW COLUMNS FROM transactionlist;
+-------------------+------------------+------+-----+---------+----------------+
| Field             | Type             | Null | Key | Default | Extra          |
+-------------------+------------------+------+-----+---------+----------------+
| TransactionNumber | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
| WagerId           | int(11)          | YES  | MUL | 0       |                |
| TranNum           | int(11)          | YES  | MUL | 0       |                |
| TranDateTime      | datetime         | NO   |     | NULL    |                |
| Amount            | double           | YES  |     | 0       |                |
| Action            | smallint(6)      | YES  |     | 0       |                |
| Uid               | int(11)          | YES  |     | 1       |                |
| AuthId            | int(11)          | YES  |     | 1       |                |
+-------------------+------------------+------+-----+---------+----------------+
8 rows in set (0.00 sec)

TranDateTime is used to save the date and time of a transaction as it happens

My Table has over 1,000,000 records in it and the statement

SELECT * FROM transactionlist where date(TranDateTime) = '2008-08-17' 

takes a long time.

EDIT:

Have a look at this blog post on "Why MySQL’s DATETIME can and should be avoided"