I should first say I'm assuming that the end of the year property tax bill is "in arrears" and applies to the previous tax period, rather than pre-paying for the upcoming property tax period. That's how it works in most of the US, but not everywhere. If they live in an area where property taxes are billed in advance for the upcoming property tax period, then it could be a rental expense.
Cash basis accounting means that expenses are attributed to the tax year that you pay the expense, that is true. But before we can even consider that, it still always has to be a business expense. Property taxes accrued during the time a property isn't a rental wouldn't be a business expense for the rental. Changing the property from personal use to a rental doesn't change the character of past expenses, regardless of when the bill is paid.
Tax law references include IRC § 262(a) which prohibits deducting personal expenses, and IRC § 212 which defined what expenses are eligible to be deducted as business expenses.
David Orr
Tax Modern
