Operations with intervals

If you divide an interval by an interval, you get a FLOAT:

=> SELECT INTERVAL '28 days 3 hours' HOUR(4) / INTERVAL '27 days 3 hours' HOUR(4);
 ?column?
------------
 1.036866359447

An INTERVAL divided by FLOAT returns an INTERVAL:

=> SELECT INTERVAL '3' MINUTE / 1.5;
 ?column?
------------
 2 mins

INTERVAL MODULO (remainder) INTERVAL returns an INTERVAL:

=> SELECT INTERVAL '28 days 3 hours' HOUR % INTERVAL '27 days 3 hours' HOUR;
  ?column?
------------
 24 hours

If you add INTERVAL and TIME, the result is TIME, modulo 24 hours:

=> SELECT INTERVAL '1' HOUR + TIME '1:30';
 ?column?
------------
 02:30:00