From what I've deduced, vacant periods of any time after the property is placed in service, as well as vacant periods between renters do not count against you. The only thing that counts against you is days of personal use "after" the property is converted to a rental.
But there's one thing I've also noticed. For those that place a rental in service close to the end of the tax year, say the last week of december for example, and don't actually get it rented out until the next year, the program will not accept zero days rented. You have to enter at least 1 day rented for it to work. In this scenario, reporting $0 income for the year is not an issue, assuming the first month's rent was not paid until after the end of the year.
