Figuring out if SQL is slow because of date manipulation into YYYYMMDD format and whats the best way to approach this.

Converting MDDYY to YYYYMMDD.sql

--TRANSDATE is Decimal(6,0) with no leading zero in MDDYY format in a 4 million record file
--i.e. 10199, 10116, 120816
WITH TABLE_TRANSACTION_WITH_YYYYMMMD_DATE as 
(
  SELECT a.*,
  CASE 
    WHEN SUBSTR(DIGITS(TRANSDATE),5,2)>70 THEN '19' || SUBSTR(DIGITS(TRANSDATE),5,2) || SUBSTR(DIGITS(TRANSDATE),1,4)  
    ELSE '20' || SUBSTR(DIGITS(TRANSDATE),5,2) || SUBSTR(DIGITS(TRANSDATE),1,4)  
  END AS TRANSDATEYYYYMMDD 
  FROM MYLIB.TRANSACTIONS a
)
SELECT * 
FROM TABLE_TRANSACTION_WITH_YYYYMMMD_DATE 
WHERE TRANSDATEYYYYMMDD >= 20140101 AND
      TRANSDATEYYYYMMDD <= 20151231