How to prove you e-filed before the deadline
Let's say you e-filed your return 2 minutes before the midnight filing deadline, but your return didn't get accepted until 2 days after the filing deadline had passed. Would your return be considered on-time or late?
The answer is that your return is on-time because you transmitted it before the deadline. You can verify that with an electronic postmark from the Intuit Electronic Filing Center.
The postmark is a time stamp that shows when your return was transmitted for e-filing, and can be used to prove that you filed your return on time, even if the government agency received your return after the filing deadline.