date() indexing error..

Using PostgreSQL 7.0 I'm Doing ...

ipa2=# CREATE INDEX "app_stat_month" on applicant_stats(date(month));
ERROR: SQL-language function not supported in this context.

ipa2=# \d applicant_stats
Table "applicant_stats"
Attribute | Type | Modifier
-----------+-----------+----------
app_id | integer |
month | date |
user_id | integer |
view_time | timestamp |
Index: app_id

When I try and do the above there is one record in the database.

ipa2=# select * from applicant_stats;
app_id | month | user_id | view_time
--------+------------+---------+------------------------------
56941 | 05-26-2000 | 291 | Fri May 26 09:19:41 2000 EDT
(1 row)

If I erase it I get the same error, only when I try and insert data into the
table..

I'm doing these kind of indexes on several tables, this is the only one I'm
having this occur.. Any ideas?

Thanks!

-Mitch