Default values in function, arguments dependency, PostgreSQL
Maybe using Postgres' support of polymorphism might be of use here.

First create the base function with both parameters.

CREATE FUNCTION years_compare(IN year1 integer,
                              IN year2 integer)
                RETURNS integer
AS
$$
BEGIN
  RETURN 1; --&lt;replace by some calculated value&gt;
END;
$$
LANGUAGE PLpgSQL;


Then create one function with only one parameter calling the base function with the first parameter and the calculated second parameter.

CREATE FUNCTION years_compare(IN year1 integer)
                RETURNS integer
AS
$$
BEGIN
  RETURN years_compare(year1, year1 - 1);
END;
$$
LANGUAGE PLpgSQL;


Finally create a parameterless version of the function calling the second one with a calculated parameter.

CREATE FUNCTION years_compare()
                RETURNS integer
AS
$$
BEGIN
  RETURN years_compare(date_part('year'::text, ('now'::text)::date)::integer);
END;
$$
LANGUAGE PLpgSQL;


Then you have three versions of the function with zero to two parameters.