I am now doing like this:

(from http://www.volny.cz/iprenosil/interbase/ip_ib_code_timestamp.htm)

CREATE PROCEDURE Is_LeapYear (Y INTEGER) RETURNS (LY INTEGER) AS
BEGIN
IF ( 60 = EXTRACT(YEARDAY FROM CAST(Y || '-3-1' AS TIMESTAMP)) ) THEN
LY = 1; /* leap year */
ELSE
LY = 0; /* normal year */
END


My procedure:

CREATE PROCEDURE KPV_AASTA (
A TIMESTAMP
) RETURNS (
L TIMESTAMP
) AS
declare variable ADay varchar(2);
declare variable AMonth varchar(2);
declare variable AYear Integer;
BEGIN

ADay = EXTRACT(DAY from A);
AMonth = EXTRACT(MONTH from A);
AYear = EXTRACT(YEAR from A);
AYear=AYear+1;

IF ((Is_LeapYear(AYear)=1) AND (AMonth=2) AND (ADay=29)) THEN ADay=28;

L = CAST(CAST(ADay as VARCHAR(2))||'.'||
CAST(AMonth as VARCHAR(2))||'.'||
CAST(AYear as VARCHAR(4)) as DATE)-1;

suspend;
END

I get error: FUNCTION UNKNOWN IS_LEAPYEAR

what may be wrong?