Changed Data Capture error

Please help. I am having this error:

[Query:QryCDC]

Invalid WHERE clause. Additional information: <Cannot compare expression <ODS_CUSTOMER.CUST_TIMESTAMP> with expression <$GV_ENDTIME>. Their types are not compatible. Ensure the types of the operands match.>. (BODI-1111078)

This is coming from an expression:

(ODS_CUSTOMER.CUST_TIMESTAMP >= $GV_STARTTIME) and

(ODS_CUSTOMER.CUST_TIMESTAMP <= $GV_ENDTIME)

How do I maintain type compatibility. I am following the data services tutorial page 90.