Note the queries below generate the same result regardless of the timezone input:
% env | grep TZ
PGTZ=CST6CDT
TZ=CST6CDT
% psql -t -c "select version()"
PostgreSQL 8.0.0beta3 on i686-pc-linux-gnu, compiled by GCC 2.96
% psql -t -c "select '1970-01-01 00:00:00+09'::timestamp + '1089352800 seconds'::interval"
2004-07-09 06:00:00
% psql -t -c "select '1970-01-01 00:00:00+03'::timestamp + '1089352800 seconds'::interval"
2004-07-09 06:00:00
% psql -t -c "select '1970-01-01 00:00:00+01'::timestamp + '1089352800 seconds'::interval"
2004-07-09 06:00:00
Ed