Extract the year from a date

Hello!

I need to extract the year part as an number from a date that got this format yyyy-mm-dd.

I found the function xf:get-year-from-date( srcvalue ) ) but I'm not sure that it is the easiets way to do or how to use it. I try to use at as below but I only get this error:
SP2-0552: Bindningsvariabeln "GET" är inte deklarerad.

SELECT pres_name FROM president, winner
    WHERE
    pres_name = name
    AND
    (year_elected - xf:get-year-from-date(birth_date) ) > 50;

So if any one could help me I would be most thankfull!

Best regarards
Fredrik