SQL helper functions for first of month and last of month

first_and_last.sql

CREATE OR REPLACE FUNCTION last_day(timestamptz)
RETURNS timestamptz AS
$$
  SELECT (date_trunc('MONTH', $1) + INTERVAL '1 MONTH - 1 day');
$$ LANGUAGE 'sql' IMMUTABLE STRICT;

CREATE OR REPLACE FUNCTION first_day(timestamptz)
RETURNS timestamptz AS
$$
  SELECT date_trunc('MONTH', $1);
$$ LANGUAGE 'sql' IMMUTABLE STRICT;