The following bug has been logged online:

Bug reference: 1749
Logged by: Nick Johnson
Email address: arachnid (AT) notdot (DOT) net
PostgreSQL version: 8.0.3
Operating system: FreeBSD
Description: date_trunc('week', ...) is incorrect for some dates
Details:

numa=# SELECT date_trunc('week', '2002-12-31'::date);
date_trunc
------------------------
2001-12-31 00:00:00-05
(1 row)

Obviously this isn't the expected behaviour...

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