No, it’s not a problem. A large number of people e-file on the last day, so IRS processing times will be longer than usual. If your extension has not been offically IRS-approved by midnight on the due date, that does not mean your request has been rejected. In fact, the IRS provides a few extra days for your tax extension to be approved. So in the rare case that your extension is temporarily rejected, you still have some time to correct any mistakes and resubmit.