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