As long as you transmitted your return before 11:59 PM on April 15, 2019, your return is considered timely filed even if it doesn't get accepted until after the deadline.
Your Electronic Postmark is a way you can prove that you e-filed before the deadline. It's a time stamp that shows when your return was transmitted. Here's how to print out a hard copy of your Electronic Postmark:
TurboTax Online – Simply go through the steps to check your e-file status online to retrieve your filing status history. You can then capture a screenshot or print it out from your web browser.
TurboTax CD/Download software
First, open your return.
Then, from File menu in TurboTax, select Check E-file Status.
You have clicked a link to a site outside of the TurboTax Community. By clicking "Continue", you will leave the Community and be taken to that site instead.