An estimated tax penalty has nothing to do with when you filed your tax return or when you paid the tax that was due on the return. Estimated tax is the quarterly payments that you make during the year if your withholding is too low or you have significant income from which no tax is withheld. You incurred an estimated tax penalty because you did not pay enough estimated tax during the year, or did not pay it early enough in the year. Or maybe you didn't make any estimated tax payments at all. The result of the insufficient estimated tax payments was that the amount you owed when you filed your tax return was more than the level at which estimated tax payments are required.
