BUG #2403: Date arithemtic using INTERVAL in UPDATE command does not work

The following bug has been logged online:

Bug reference:      2403
Logged by:          Harry E. Clarke
Email address:      Harry.Clarke@metrosky.co.uk
PostgreSQL version: 8.1.3
Operating system:   Suse Linux 10.0
Description:        Date arithemtic using INTERVAL in UPDATE command does
not work
Details:

Entering the command

UPDATE table SET col_date = col_date - INTERVAL '100' YEAR where condition;

does not perform any date aritimetic, and thus the date value in col_date
remains unchanged. col_date contains a value such as '2039-12-07'. If the
command

UPDATE table SET col_date = DATE '1939-12-07' where condition;

is entered, the command executes as expected.