Interval+tz-tz gives unexpected result

dev=# select '0 sec'::interval+'2002/10/21 09:48'::timestamptz-'2002/10/21
09:30'::timestamptz;
 ?column?
----------
 -09:30
(1 row)

I would have expected '00:18'

dev=# select version();
                           version
--------------------------------------------------------------
 PostgreSQL 7.2.3 on i686-pc-cygwin, compiled by GCC 2.95.3-5
(1 row)

Thank you,
- Stuart