Related to this E8 topic.
Reduce plan price based on certain date of purchase.
Mostly applicable to timeframe plans (till the end of current month/year ... etc.)
Typical usecase & scenario
A local sports-club has memberships for a full calendar-year. i.e. 2013
No matter when I joined the club, my membership expires on Dec 31th of the current year.
In addition, I should get a reduced price, if I joined in the second half of the current year. i.e. I join in summer/Juli 2013, I should only pay 50% of normal plan price.
Just one example. Maybe the date check and amount can be flexible.