select datediff( year, cast('27.07.1990' as date), cast('01.01.2024' as date)),
       dateadd( year, 34, cast('27.07.1990' as date))
from RDB$DATABASE

select datediff( year, :DATE_BORN, CURRENT_DATE) -
       iif( dateadd( year, (datediff( year, :DATE_BORN, CURRENT_DATE)), :DATE_BORN) > CURRENT_DATE, 1, 0)
from RDB$DATABASE