Postgresql function to calculate the months between two dates.

months_between.sql

CREATE OR REPLACE FUNCTION months_between(
    date,
    date)
  RETURNS integer AS
$BODY$  
DECLARE
  dt_final date;
BEGIN
  dt_final := $2;
  
  IF $1 = date_trunc('MONTH', $1)::date AND $2 = last_day_of_month($2) THEN
    -- É um mês inteiro
    dt_final := dt_final + interval '1 day';
  END IF;
  
  RETURN abs(months_of(age($1, dt_final)));
END; $BODY$
  LANGUAGE plpgsql;

months_of.sql

CREATE OR REPLACE FUNCTION months_of(interval)
  RETURNS integer AS
$BODY$
  SELECT extract(years from $1)::int * 12 + extract(month from $1)::int
$BODY$
  LANGUAGE sql;

where is last_day_of_month