SET TERM ^ ;

CREATE OR ALTER PROCEDURE TEST (
    UNIT INTEGER)
RETURNS (
    RESULT DATE)
AS
DECLARE VARIABLE START_DATE DATE;
DECLARE VARIABLE START_VALIDITY INTEGER;
DECLARE VARIABLE END_VALIDITY INTEGER;
BEGIN
  START_DATE = '01/01/2021';
  START_VALIDITY = 5;
  END_VALIDITY = 10;

  RESULT =
    CASE
      WHEN UNIT = 4 -- Day
        THEN DATEADD((END_VALIDITY - START_VALIDITY + 1) DAY TO START_DATE)
      WHEN UNIT = 5 -- Week
        THEN DATEADD((END_VALIDITY - START_VALIDITY + 1) WEEK TO START_DATE)
      WHEN UNIT = 7 -- Month
        THEN DATEADD((END_VALIDITY - START_VALIDITY + 1) MONTH TO START_DATE)
    END;

  SUSPEND;
END^

SET TERM ; ^

/* Existing privileges on this procedure */

GRANT EXECUTE ON PROCEDURE TEST TO SYSDBA;