I guess the rental could have started in Dec
There's really no guessing here. The property is "in service" on the first day the property is available for rent - meaning that if on Dec 1 I walked up to you with the deposit, first months rent and last months rent and you accepted it (or would have accepted it) with the understanding I was moving in that very day, then the property was available for rent on Dec 1 of the tax year. Doesn't matter if it actually sat empty from the date it was "available for rent", through the end of the tax year. Generally, the day the property is considered "in service" and available for rent, is the day you put the FOR RENT sign in the front yard.
