How to create an index for type timestamp column using rtree?

Hi all,

I'm storing some timestamps as integers (UTF) in a table and I want to
query by <= and >= for times between a certain period. The table has
gotten quite large and is now very slow in querying.

I think it's time to create an index for the timestamp column.

I tried using an rtree (for <= and >= optimization):

create INDEX logs_timestamp ON logs using rtree (timestamp);

but I get

ERROR: data type integer has no default operator class for access
method "rtree"
You must specify an operator class for the index or define a
default operator class for the data type

Do I have to create an rtree type for my timestamp integer column?

Existing rtree columns are below.

Pls help.

Thanks,
Chris

server=> select am.amname as acc_method, opc.opcname as ops_name from
pg_am am, pg_opclass opc where opc.opcamid = am.oid order by
acc_method, ops_name;
acc_method | ops_name
------------+-----------------
btree | abstime_ops
btree | bit_ops
btree | bool_ops
btree | bpchar_ops
btree | bytea_ops
btree | char_ops
btree | cidr_ops
btree | date_ops
btree | float4_ops
btree | float8_ops
btree | inet_ops
btree | int2_ops
btree | int4_ops
btree | int8_ops
btree | interval_ops
btree | macaddr_ops
btree | name_ops
btree | numeric_ops
btree | oid_ops
btree | oidvector_ops
btree | text_ops
btree | time_ops
btree | timestamp_ops
btree | timestamptz_ops
btree | timetz_ops
btree | varbit_ops
btree | varchar_ops
hash | bpchar_ops
hash | char_ops
hash | cidr_ops
hash | date_ops
hash | float4_ops
hash | float8_ops
hash | inet_ops
hash | int2_ops
hash | int4_ops
hash | int8_ops
hash | interval_ops
hash | macaddr_ops
hash | name_ops
hash | oid_ops
hash | oidvector_ops
hash | text_ops
hash | time_ops
hash | timestamp_ops
hash | timestamptz_ops
hash | timetz_ops
hash | varchar_ops
rtree | bigbox_ops
rtree | box_ops
rtree | poly_ops
(51 rows)