Undocumented datetime functions

The following date/time related functions are not documented in the
obvious place for them, namely
http://www.postgresql.org/devel-corner/docs/postgres/functions-datetime.htm

now()
CURRENT_DATE
CURRENT_TIME
CURRENT_TIME(n)
CURRENT_TIMESTAMP
CURRENT_TIMESTAMP(n)
EXTRACT(part FROM datetime)    --- ANSIfied version of date_part

BTW, it seems that date_part accepts rather more options than are
documented on this page, as well.

            regards, tom lane