Found this question on sfdc99.com as an example of possible Salesforce developer interview questions. Does anyone know how this can be done? Of course, without coding.
You could create a text field on Opportunity that is marked as "unique" and doesn't allow duplicate values. Then use a Workflow Rule and Field Update to update the field with the account/date combination eg:
AccountId & "-" & YEAR(CreatedDate) & "-" & MONTH(CreatedDate) & "-" & DAY(CreatedDate)
The workflow criteria would be
true so that it fired all the time.