Replacing NULL values with one string from provided list
You can use something like

SELECT Id,
       ISNULL(Date, (SELECT TOP 1 *
                     FROM   (VALUES(2015),
                                   (2012)) R(Ryear)
                     ORDER  BY CRYPT_GEN_RANDOM(DATALENGTH(Id))))
FROM   YourTable 


The reference to Id in the ORDER BY is just to make it correlated so that SQL Server is less likely to spool the result and replay it for multiple rows.
Select a column that is unique for this.

BTW: There is a CHOOSE function that looks tempting but when this gets expanded out to CASE the random number function gets copied out too and so this is not suitable for the task.

Don't use this

SELECT Id,
       CHOOSE(1 + ABS(CRYPT_GEN_RANDOM(4) % 2),2015,2012)
FROM   YourTable 


Because it is evaluated as

CASE
  WHEN (1 + ABS(CRYPT_GEN_RANDOM(4) % 2)) = 1
    THEN 2015
  ELSE
    CASE
      WHEN (1 + ABS(CRYPT_GEN_RANDOM(4) % 2)) = 2
        THEN 2012
      ELSE NULL
    END
END 


(And CASE ABS(CRYPT_GEN_RANDOM(4) % 2) WHEN 0 THEN 2012 WHEN 1 THEN 2015 END would have the same problem)