But see
https://www.irs.gov/newsroom/irs-advisory-prepaid-real-property-taxes-may-be-deductible-in-2017-if-assessed-and-paid-in-2017
So is the IRS clearly saying that for example in New Jersey, even though Christie signed a executive order to allow you to pay for the entire 2018, you will only be allowed to count the first 2 quarters, since in most municipalities, they only actually 'assessed' the first 2 quarters?
Example 2: County B also assesses and bills its residents for property taxes on July 1, 2017, for the period July 1, 2017 – June 30, 2018. County B intends to make the usual assessment in July 2018 for the period July 1, 2018 – June 30, 2019. However, because county residents wish to prepay their 2018-2019 property taxes in 2017, County B has revised its computer systems to accept prepayment of property taxes for the 2018-2019 property tax year. Taxpayers who prepay their 2018-2019 property taxes in 2017 will not be allowed to deduct the prepayment on their federal tax returns because the county will not assess the property tax for the 2018-2019 tax year until July 1, 2018.