While that's generally the way such deadlines are handled under the law, I would not depend on it when it comes to FinCEN's filing deadlines. I would instead build procedures such that filing would be completed by the 10th calendar day to prevent having any potential problems meeting the 15-day deadline. If you absolutely need to know the answer, I'd recommend asking FinCEN. Document the call and whom you talk with. You'll need that information if your auditor or examiner challenges the filing as late.