Index on timestamp to date field

I have a timestamp field where I find I'm doing a lot of searching by date
(YYYY-MM-DD) or using this field as a match to another table that has a
date format.  I wanted to create an index on the  timestamp field using a
date format.  Is this possible?  I tried:
 CREATE INDEX test_2 ON table1 USING btree to_char(field2, 'MM-DD-YYYY');
but I get an error message error at or new to_char

TIA

Patrick Hatcher
Macys.Com
Legacy Integration Developer
415-422-1610 office
HatcherPT - AIM