REPLACE FUNCTION next_cm(now TIMESTAMP WITH TIME zone, months INTEGER) RETURNS TIMESTAMP WITH TIME zone AS $$
<< outerblock >>
DECLARE
    m_offset INTEGER;
BEGIN
    m_offset := months;
    IF (get_cm(now) + ('17:00'::INTERVAL)) < now THEN
        m_offset := m_offset + 1;
    END IF;
    RETURN get_cm(date_trunc('month',now) + (m_offset||'month')::INTERVAL) + ('16:30'::INTERVAL);
END;
$$ LANGUAGE plpgsql;

CREATE OR REPLACE FUNCTION get_cm(now TIMESTAMP WITH TIME zone) RETURNS TIMESTAMP WITH TIME zone AS $$
<< outerblock >>
DECLARE
BEGIN
    RETURN date_trunc('month', now) + ((((8 + 4 - date_part('isodow',date_trunc('month', now)))::integer%7) + 14)||'days')::INTERVAL;
END;
$$ LANGUAGE plpgsql;


-- Ergebnis:

# SELECT next_cm(now(),0), next_cm(now),1);
         next_cm         |        next_cm
------------------------+------------------------
 2011-07-15 16:30:00+02 | 2011-08-19 16:30:00+02
(1 ROW)