I would use the Table Value Constructor version instead of the standard UNPIVOT, because it's more flexible and you need that flexibility here.  I've used a sample where the StartDate is in a datetime format, you can replace that with whatever you're using to handle the dates.

SELECT ContractName, DATEADD(MONTH, n, StartMonth), [Target]
FROM tblTarget
CROSS APPLY
(
    VALUES
        (0, TargetMonth1),
        (1, TargetMonth2),
        (2, TargetMonth3),
        (11, TargetMonth12)
) v(n, [Target])

Drew