I'm pretty sure the interval values are buried in the code, but there is nothing to prevent you from creating your own reference table. :)
CREATE TABLE time_intervals
(
time_interval_name varchar(15) NOT NULL,
CONSTRAINT time_intervals_pk PRIMARY KEY (time_interval_name)
);
INSERT INTO time_intervals
VALUES
('microsecond'),
('millisecond'),
('second'),
('minute'),
('hour'),
('day'),
('week'),
('month'),
('year'),
('decade'),
('century'),
('millennium');
SELECT * FROM time_intervals;