How do you limit number of downloads of a book?

The Store owner can decide on number of allowed downloads for all books in the Store. 

Here is how it works.

First of all we have cycles of 7 days which we call week, 30 days which we call month, and 365 days that we call year. They all starts from the day the first download has occurred.

A Store owner (admin) can limit number of downloads per week, month, year. Zero means unlimited.