Do you mean adding a field to the Customer Info object and populating it with a calculated date that is based on the contract start date?
If so, the rule would look something like this:
1. Pull dataset of all contracts with contract start date and account SFDC ID.
2. Transform the dataset to create a calculated "Renewal Date" field of Contract Created Date - 365 (or 366 for leap year). (This assumes you want to populate the previous renewal date. Add 365 or 366 if you want to populate the NEXT renewal date).
3. Use Load to SFDC Object and target the Customer Info object "Renewal Date" field with the calculated field from step 2.
4. Pull this "Renewal Date" field into the C360 (or other location as desired)
Let me know if you have any other questions? Thanks!