The official validity starts on the day the application gets approved. So, if say you applied in January 2020, and your application was approved for three years in April 2020, your PGWP will be valid from 04-2020 up to 04-2023.
Obviously, you would have had a nice little 3 months cushion between when you applied and when your application is approved where you would have been able to work full time or apply to jobs.