BUG #5031: DATE_TRUNC returns the wrong value when specifying MONTH

The following bug has been logged online:

Bug reference:      5031
Logged by:          Mark Douglas
Email address:      mark@steelhousemedia.com
PostgreSQL version: 8.4.0
Operating system:   Ubunto Linux
Description:        DATE_TRUNC returns the wrong value when specifying MONTH
Details:

The following use of DATE_TRUNC returns the wrong value. I called the
function on 2009-09-02. It should return '2009-09-01 00:00:00' for the
following usage:

SELECT DATE_TRUNC('MONTH', CURRENT_DATE);

It instead returns '2009-08-31 17:00:00.

Casting CURRENT_DATE to a TIMESTAMP causes it to return the correct value
but that shouldn't be required. Cast example:

SELECT DATE_TRUNC('MONTH', CAST(CURRENT_DATE AS TIMESTAMP));