The extension was for more time to file the return but the estimated taxes owed were due back in April. So if you didn't pay the balance due until you filed you do owe interest for the late payment and a possible failure to pay penalty and the IRS/state each bills you for what they calculate you owe based on when they get the payment. The TT program only calculates the estimated underpayment penalty until the 4/15 filing deadline.
