I had to use a CONVERT with the SUBSTRING Function as opposed to the CAST.

This works.

SELECT

CASE WHEN CAST(LEFT(Customer.STARTDATE, 2 AS VARCHAR(2)))

= '98' THEN CAST(substring(Customer.STARTDATE, 3, 2) AS VARCHAR(2))

WHEN LEFT(CAST(Customer.STARTDATE, 2) AS VARCHAR(2)) = '99' THEN

SUBSTRING(CONVFERT(VARCHAR(20),(Customer.STARTDATE, 3, 2) AS VARCHAR(2)) END AS LosstMo

FROM Customer

Thanks.