Simple, really ... using v7.2b3 that hasn't been released yet ...
traf_stats=# select EXTRACT(WEEK FROM TIMESTAMP runtime) from hourly_stats;
ERROR: parser: parse error at or near "runtime"
traf_stats=# \d hourly_stats Table "hourly_stats" Column | Type | Modifiers
------------+-----------------------------+-----------from_ip | inet |to_ip | inet
|port | integer |bytes | bigint |runtime |
timestamp(6)with time zone |no_records | integer |
Indexes: hourly_from_ip, hourly_to_ip