I added a new user to my existing subscription, but my subscription end date shifted by less than a year. What happened?
Every user in a subscription must have the same end date. Adding a user license to an existing subscription will license the new user through the original expiration date of the subscription. Any additional time will be applied to all existing user licenses, and will extend the subscription’s overall end date.
For example, customer has an Office 365 subscription with 100 seats. However, in month 10, customer adds 100 additional seats. The expiration date for all 200 seats is now 5 months past the expiration date of the first 100 seats. The calculation is as follows: (100 seats x 10 months) / 200 seats