Payment Module Installed - Configured - but doesn't work - "Error (97) This transaction cannot be accepted"

This project is not going to take long to complete - somewhere between 15 and 30 minutes, I expect. My VPS is running CentOS 6 and the X-cart 4 shopping cart. I have installed the payment module for "[login to view URL] - SIM" and it is configured with valid information in the API Login ID, Transaction key, and other fields ... the only field I have doubts about is the one labeled as follows: The difference between your server time and the [login to view URL] server time (in seconds)*: The asterisk (*) at the end of the field name corresponds to the following: * Please ensure your system clock is set to proper time and time zone. AuthorizeNet gateway will validate the timestamp of a fingerprint submitted to the system before accepting a transaction. The gateway will only accept a fingerprint generated within an hour before the transaction was received. If the fingerprint has expired, the transaction will be rejected. The acceptance of the transaction by the gateway is therefore dependent on the merchant's system clock being set to the correct time based on the time zone setting of the system. Though it is not required that the system clock be accurate to the last second, it is recommended that the clock is as close as possible to the actual time for the merchant's time zone. If you can not adjust system time on your server you can specify the difference between your server time and the [login to view URL] server time in the corresponding field above. When I attempt to checkout I end up seeing a white page containing only the following: The following errors have occurred. (97) This transaction cannot be accepted.