The Salesforce Opportunity Close Date should always be in the future, and not be in the past, since this will result in your Salesforce pipeline report being inaccurate.
Create a validation rule in Salesforce to enforce that the close date cannot be in the past.
The Salesforce Opportunity Close Date is your best estimate of when you expect to close the sale.
The Close Date will change as you work the sale. It should never be set in stone as it affects your sales pipeline, and it should never be in the past.
What Close Dates in the past are good for is signalling which sales people are not staying on top of their opportunities!
