As xmasbaby0 suggests, if you underpaid for earlier quarters of the tax year or by April 15 you did not pay the balance due, you'll still owe a penalty and interest on the underpayment even though your tax return might be considered timely filed under the circumstances. You can check your Tax Account Transcript to see when estimated tax payments and payment of the balance due were applied to your tax account: https://www.irs.gov/individuals/get-transcript
