what time does the period starts counting, they day the license was purchase or the day the license was activated?
This logic works on Customer created date. This is date when customer account was created in Connect. The customer gets created in Connect when first Order gets provisioned for that customer (A customer is recognized by Name and Fulfillment Contact). This is at the time of activating their first activation code. When new licenses are added, license moves value is copied from Customer configuration of created customer and same concept of moves is followed for those licenses as well.