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

select trunc((datediff( day, cast('27.07.1990' as date), CURRENT_DATE) + 1) / 365.25)
from RDB$DATABASE

create or alter function GET_DATE_PART (
    D1 date,
    D2 date,
    VID integer = 0)
returns integer
as
declare variable y int;
declare variable d01 date;
declare variable d02 date;
begin
  d01 = minvalue(d1, d2);
  d02 = maxvalue(d1, d2);

  if (vid = 0) then -- Дни
    y = d02 - d01 + 1;
  else
  if (vid = 1) then -- Месяцы
    y = datediff(month, d01 - extract(day from d01) + 1, d02 - extract(day from d01) + 2);
  else
  if (vid = 2) then -- Годы
  begin
    y = extract(year from d02) - extract(year from d01);
    if (dateadd(year, y, d01) > d02) then
      y = y - 1;
  end

  if (d2 < d1) then y = -y;
  return y;
end

create function calculate_age(
    birthday date
  , otherday date
)returns
    integer
as
    declare variable birthyear integer;
    declare variable otheryear integer;
    declare variable age       integer;
begin

    birthyear = extract( year from birthday );
    otheryear = extract( year from otherday );
    age       = otheryear - birthyear;

    if(          extract( month from birthday ) > extract( month from otherday ) )then begin
        age = age - 1;
    end else if( extract( month from birthday ) = extract( month from otherday ) )then begin
        age = iif( extract( day from birthday ) > extract(   day from otherday ), age - 1, age );
    end

    return age;
end