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;

SET TERM ^;
CREATE OR ALTER PROCEDURE SUBYEAR
  (
    TMSTMP  TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
    CNT     SMALLINT DEFAULT 1
  )
  RETURNS
  (
    RET     TIMESTAMP
  )
AS
  DECLARE VARIABLE T NUMERIC(14, 2) DEFAULT NULL;
  DECLARE VARIABLE Y INTEGER DEFAULT NULL;
BEGIN
  IF (:CNT > 0) THEN
    BEGIN
      IF (:CNT > 1) THEN
        SELECT RET - 365
          FROM SUBYEAR(:TMSTMP, :CNT - 1)
          INTO: RET;
      ELSE
        RET = TMSTMP - 365;
      Y = EXTRACT(YEAR FROM :RET);
      T = CAST(Y AS NUMERIC(14, 2)) / 4 - Y / 4;
      IF (:T = 0 AND RET < CAST(Y || '-12-31 00:00' AS TIMESTAMP)) THEN
        RET = RET - 1;
    END
  SUSPEND;
END^
SET TERM ;^
COMMIT WORK;

CREATE GLOBAL TEMPORARY TABLE TEST_SUBYEAR
(
    SRC   TIMESTAMP NOT NULL,
    XPECT TIMESTAMP NOT NULL,
    CNT   SMALLINT  DEFAULT 1 NOT NULL,
  CONSTRAINT TEST_SUBYEAR PRIMARY KEY (SRC, CNT)
) ON COMMIT DELETE ROWS;
COMMIT WORK;


EXECUTE BLOCK
  RETURNS
  (
    ACTSTR  VARCHAR(78),
    XPECTD  TIMESTAMP,
    RESULT  TIMESTAMP,
    STATUS  CHAR(4)
  )
AS
BEGIN
  INSERT INTO TEST_SUBYEAR VALUES ('2016-01-01 00:00', '2015-01-01 00:00', 1);
  INSERT INTO TEST_SUBYEAR VALUES ('2016-01-01 00:00', '2012-01-01 00:00', 4);
  INSERT INTO TEST_SUBYEAR VALUES ('2016-02-28 00:00', '2015-02-28 00:00', 1);
  INSERT INTO TEST_SUBYEAR VALUES ('2016-02-28 00:00', '2012-02-28 00:00', 4);
  INSERT INTO TEST_SUBYEAR VALUES ('2016-02-29 00:00', '2015-03-01 00:00', 1);
  INSERT INTO TEST_SUBYEAR VALUES ('2016-02-29 00:00', '2012-02-29 00:00', 4);
  INSERT INTO TEST_SUBYEAR VALUES ('2016-03-01 00:00', '2015-03-02 00:00', 1);
  INSERT INTO TEST_SUBYEAR VALUES ('2016-03-01 00:00', '2012-03-01 00:00', 4);
  INSERT INTO TEST_SUBYEAR VALUES ('2016-12-31 00:00', '2015-12-31 00:00', 1);
  INSERT INTO TEST_SUBYEAR VALUES ('2016-12-31 00:00', '2012-12-31 00:00', 4);

  INSERT INTO TEST_SUBYEAR VALUES ('2017-01-01 00:00', '2016-01-01 00:00', 1);
  INSERT INTO TEST_SUBYEAR VALUES ('2017-01-01 00:00', '2013-01-01 00:00', 4);
  INSERT INTO TEST_SUBYEAR VALUES ('2017-02-28 00:00', '2016-02-28 00:00', 1);
  INSERT INTO TEST_SUBYEAR VALUES ('2017-02-28 00:00', '2013-02-28 00:00', 4);
  INSERT INTO TEST_SUBYEAR VALUES ('2017-03-01 00:00', '2016-02-29 00:00', 1);
  INSERT INTO TEST_SUBYEAR VALUES ('2017-03-01 00:00', '2013-03-01 00:00', 4);
  INSERT INTO TEST_SUBYEAR VALUES ('2017-12-30 00:00', '2016-12-29 00:00', 1);
  INSERT INTO TEST_SUBYEAR VALUES ('2017-12-30 00:00', '2013-12-30 00:00', 4);
  INSERT INTO TEST_SUBYEAR VALUES ('2017-12-31 00:00', '2016-12-31 00:00', 1);
  INSERT INTO TEST_SUBYEAR VALUES ('2017-12-31 00:00', '2013-12-31 00:00', 4);

  FOR
      SELECT
          'SUBYEAR(''' || SRC || ''', ' || CNT || ') == ''' || XPECT || '''',
          XPECT,
          (SELECT RET FROM SUBYEAR(SRC, CNT))
        FROM TEST_SUBYEAR
        ORDER BY SRC, CNT
        INTO: ACTSTR, XPECTD, RESULT
    DO
      BEGIN
        IF (:XPECTD = :RESULT) THEN
          STATUS = 'PASS';
        ELSE
          STATUS = 'FAIL';
        SUSPEND;
      END
END;