SELECT
    CURRENT_TIMESTAMP
    - Iif(Mod(Extract(YEAR FROM CURRENT_DATE), 4) = 0, 366, 365)
  FROM RDB$DATABASE;

SET TERM ^;
CREATE OR ALTER PROCEDURE SUBYEAR
  (
    TMSTMP  TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
    CNT     SMALLINT DEFAULT 1
  )
  RETURNS
  (
    RET     TIMESTAMP
  )
AS
  DECLARE VARIABLE Y INTEGER DEFAULT NULL;
  DECLARE VARIABLE D SMALLINT DEFAULT 365;
BEGIN
  IF (:CNT > 0) THEN
    BEGIN
      IF (:CNT > 1) THEN
        SELECT RET
          FROM SUBYEAR(:TMSTMP, :CNT - 1)
          INTO: RET;
      ELSE
        RET = TMSTMP;
      SELECT EXTRACT(YEAR FROM :RET)
        FROM RDB$DATABASE
        INTO: Y;
      IF (:RET < Cast('01.03.' || :Y AS TIMESTAMP)) THEN Y = Y - 1;
      IF (CAST(:Y AS NUMERIC(14, 2)) / 4 = :Y / 4) THEN D = 366;
      RET = RET - D;
    END
  SUSPEND;
END^
SET TERM ;^
COMMIT WORK;