How to create a multi-column index with 2 dates using 'gist'?

Hi,

I am using 8.3 and pgAdmin III. I have a couple of tables using 2 DATE
columns like 'startdate' and 'enddate' (just date, not interested in time in
these columns). I have some queries (some using OVERLAPS) involving both
'startdate' and 'enddate' columns. I tried to create a multi column index
using pgAdmin and it comes back with this error:

ERROR: data type date has no default operator class for access method "gist"
HINT: You must specify an operator class for the index or define a default
operator class for the data type.

I search the pdf docs and online without finding what an "operator class"
for DATE would be. Would a multi-column index help in that case (OVERLAPS
and dates comparison) anyway? Or should I just define an index for each of
the dates?

Below are the table and index defintions.

Thanks

Fred

---------------------------------------------
CREATE INDEX startenddate
ON times USING gist (startdate, enddate);

---------------------------------------------
-- Table: times

-- DROP TABLE times;

CREATE TABLE times
(
id serial NOT NULL,
startdate date NOT NULL,
enddate date NOT NULL,
starttime time without time zone,
endtime time without time zone,
CONSTRAINT pk_id PRIMARY KEY (id)
)
WITH (OIDS=FALSE);
ALTER TABLE times OWNER TO postgres;
GRANT ALL ON TABLE times TO postgres;
GRANT ALL ON TABLE times TO public;