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

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.