Estimated tax payments are only credited when actually paid. However, by default, your income is treated as having been received evenly throughout the year. When you receive a large amount of income late in the year and make the corresponding estimated tax payment late in the year, the default treatment results in underpayment of taxes for the first tax quarters of the year. To allow the income to be treated as received when actually received instead of received evenly throughout the year., you must annualize the income on Schedule AI of Form 2210. TurboTax will ask you if you want to annualize income.