Each product key provides users with 12 months of service from today. Since your existing users have an early renewal date, ...

Each product key provides users with 12 months of service from today. Since your existing users have an early renewal date, we have averaged the two dates (see Calculation) to make it easier to manage your subscription with a single end date for all of your users. No time is added or lost during the calculation.
Calculation:
[A (initial users) x B (time remaining on subscription) + C (new users) x (12 months) ] ÷ total number of users = new time remaining