Thanks for everybody. I got it to work :)

procedure is now following:

CREATE PROCEDURE SP_PUHKUS (
BEGIN_DATE TIMESTAMP,
END_DATE TIMESTAMP
) RETURNS (
WORKED_MONTHS INTEGER
) AS

declare variable ADay Integer;
declare variable AMonth Integer;
declare variable AYear Integer;

declare variable LDay Integer;
declare variable LMonth Integer;
declare variable LYear Integer;
declare variable years Integer;
declare variable months Integer;

BEGIN

ADay = EXTRACT(DAY from begin_date);
AMonth = EXTRACT(MONTH from begin_date);
AYear = EXTRACT(YEAR from begin_date);

LDay = EXTRACT(DAY from end_date);
LMonth = EXTRACT(MONTH from end_date);
LYear = EXTRACT(YEAR from end_date);

years=LYear-AYear;
months=LMonth-AMonth;
worked_months=(years*12)+months;

If (LDay<15) then worked_months=worked_months-1;
If (ADay<=15) then worked_months=worked_months+1;
If ((ADay<=15) and (LDay<15) and (AMonth<>LMonth) and (AYear<>LYear)) then worked_months=worked_months+1;



SUSPEND;
END