Your new use year starts at the beginning of your use year month, which is August 1.
Since August 1 in the coming year falls in the calendar year of 2019, and that is when your use year starts, it is the 2019 use year. Any reservations made for the next 12 months after that will be in the 2019 use year. Even if you use those points to make a reservation in February 2020 or anytime in 2020 up until the end of July 2020 then you are still in your August 2019 use year.
The only way to get 2018 points into your 2019 use year, so you could use them, is to bank them. And you must do this by FOUR months before the end of your youth year. This means that they must be banked by March 31, 2019.
When you are making reservations in the 2019 use year, even if you are making those reservations now in 2018, you will be in the 2019 future. Because you are making the reservation for the 2019 use year, you will be able to borrow points from 2020, if you need them.