CREATE TABLE Tab ( 
    Id   INTEGER  PRIMARY KEY,
    Date DATETIME,
    Srok INT 
);

INSERT INTO Tab values (1, '2013-01-01', 6);
INSERT INTO Tab values (2, '2013-02-01', 12);
INSERT INTO Tab values (3, '2013-03-01', 24);

SELECT 
    Id,
    Date,
    Srok,
    datetime(Date,'+12 months') as EndDate
FROM
    Tab