In table 9-27 of the 9.2 docs:
http://www.postgresql.org/docs/9.2/interactive/functions-datetime.html#FUNCTIONS-DATETIME-TABLE
There is an entry for date_trunc(text, timestamp) but no entry for date_trunc(text, interval). The full docs for date_trunc:
http://www.postgresql.org/docs/9.2/interactive/functions-datetime.html#FUNCTIONS-DATETIME-TRUNC
do mention both the timestamp and interval forms of date_trunc but that's easy to miss if you're just scanning the function lists.
Eric Howe
eric(at)pieinsky(dot)ca
http://pieinsky.ca/