We have an annual membership. Instead of having a different renewal date for each member based on when (s)he joined, we want the subscriptions (i.e. recurring payments) for all members to renew on the same date each year. In order to do this, we need the ability to automatically prorate the price when someone buys in the middle of the year.
Example: Annual Membership period runs from June 1 to June 1. New member joins in December (six months into the membership period). We would like for that member to be charged $50 for the partial year, and then the full $100 for each full year after.