Estimated tax is the method used to pay tax on income that is not subject to withholding. This income includes earnings from self-employment, interest, dividends, rents, and alimony. Taxpayers who do not choose to have taxes withheld from other taxable income should also make estimated tax payments.
You can make estimated tax payments up to 12/31 the previous year because when the current year comes, the estimated tax payments made are included for the current tax year. You cannot pay your taxes after the tax year ends.
Please note that paying your liability (taxes you owed) for the previous tax year is NOT an estimated tax payment.