Estimated taxes were underpaid, documents required to complete the tax return are not available and might not be available for a long time. I want to pay based on documents I do have and my best guess for the rest to reduce the penalty for underpaid estimated taxes so I will file for an extension and make a payment. When get all of the documentation and complete my taxes, what date do I use as the date that the payment was made when I calculate the penalty?
Generally, the date of the US postmark stamped on a return, claim, statement, document, or payment that is mailed shall be deemed to be the date of delivery or the date of payment.
For EFTPS payments the payment is the date that you submit the payment.