BUG #6656: Wrong timestamptz + interval calculation

The following bug has been logged on the website:

Bug reference:      6656
Logged by:          Marcus Wirsing (Hesotech GmbH)
Email address:      mw@hesotech.de
PostgreSQL version: 9.1.3
Operating system:   Windows 7 64Bit
Description:=20=20=20=20=20=20=20=20

When I execute the statement

select timestamptz '2012-03-01 00:00:00+00' + interval '1 month'

I expect the result:
"2012-04-01 02:00:00+02"

but postgres returns:

"2012-04-01 01:00:00+02"

My local time zone is UTC+1.