Hello!
The premium Time count from the exactly hour you purchased it. For example: If you purchase your Premium Time at 15:00 CEST, the next SS when you log into your account at 10:00 CEST it will still show you have 30 days left of your Premium Time.
You also can realized when you see you have 0 days left but you still are PA, is cuz you still have some hours left from the time your purchase the first time.