try this one............

SELECT

CASE WHEN CAST(substring(Customer.STARTDATE,2,2) AS VARCHAR(2)) = '98' THEN CAST(substring(Customer.STARTDATE, 4, 2) AS VARCHAR(2))

WHEN CAST(substring(Customer.STARTDATE,2,2) AS VARCHAR(2)) = '99' THEN CAST(substring(Customer.STARTDATE, 4, 2) AS VARCHAR(2))

END AS LossMo

FROM Customer

OR

SELECT

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

WHEN CAST(left(Customer.STARTDATE,2) AS VARCHAR(2)) = '99' THEN CAST(substring(Customer.STARTDATE, 3, 2) AS VARCHAR(2))

END AS LossMo

FROM Customer