[jeltz]

No, the code casts to "timestamp without time zone" before running date_trunc so it is immutable.

For example:

  CREATE OR REPLACE FUNCTION gd_day(timestamptz, text)
    RETURNS timestamptz AS
  $$
    SELECT DATE_TRUNC('day', $1 AT TIME ZONE $2) AT TIME ZONE $2;
  $$
    LANGUAGE SQL;

EDIT: The reason that some time functions in PostgreSQL are not immutable is that they are affected by the current time zone setting of the session.