expression index on date_trunc

Hi,

is is possible to create an expression index based on the date_trunc function?

Working with PostgreSQL 8.2, I'm trying to create an index using:

  CREATE INDEX request_day_idx ON moksha_sm_request (date_trunc('day', request_received));

I get the error message:

  ERROR: functions in index expression must be marked IMMUTABLE
  SQL state: 42P17

I'd like to use the index to speed up a query that does a "group by" on the day part of a timestamp to lump all the
stuffthat happens in the same day together (I have a timestamp because the information in the table is coming from an
appserverlogfile and the date/time component of each line provides a full timestamptz (always the same tz!)). 

I'd be grateful if someone could point out what part of the statement is not IMMUTABLE or how I could mark my create
indexstatement as being immutable. 

Thanks,

Andy