Passes are only valid for one year. A new physical pass needs to be purchased after your current one has expired.
If you purchase the Annual Pass on any date in August, 2019 then your pass will be valid until August 31, 2020. When you receive your pass you will see a hole has been punched in the card on the month of August and notice the expiration year to the right says 2020, this means the pass is valid until the last day of August 2020.
Picture of the back of the annual pass (no hole has been punched in the card of the month of purchase):