In addition, to prevent households from attempting to maximize their state and local tax deductions in 2017 (before the cap takes effect in 2018), the new rules explicitly stipulate that any 2018 state income taxes paid by the end of 2017 are not deductible in 2017 (and instead will be treated as having been paid at the end of 2018). However, this restriction applies only to the prepayment of income taxes (not to property taxes), and applies only to actual 2018 tax liabilities, which means it is still permissible to pay 4th quarter 2017 estimated taxes by the end of 2017 (and not in early January of 2018) to obtain the 2017 deduction. (And in point of fact, there wasn’t much existing authority to support deducting prepayments of income taxes for a future tax year, anyway.)