This offer takes the previous payment(s) into account. What this means for you is that we sum all monthly payments that you made before buying a yearly plan and add a certain amount to those payments, which in turn makes you eligible for a 50% discount. You end up paying 50% less compared to your monthly payments.
Here is an example.
Let's assume that a monthly PRO+ subscription costs $29.95 and you paid the monthly fee 3 months in a row. You have paid $89.85 in total. If you pay $29.95 for 12 months then you'll end up paying $359.4 in total. However, the offer above allows you to use the service for 12 months and pay half of the amount ($359.4 / 2 = $179.7). You'd basically have to pay $89.85 extra as you've already paid $89.85.
Since your monthly subscription is included in the total calculation, your annual billing date is shifted accordingly. Basically, your billing date equals your monthly subscription start date and not the yearly subscription purchase date.