Count unique values on a specific date

Hello, again I need help from you guys. This time I want to count active customers at December 31st every year (2008-2013). Most customers are listed twice on specific dates, but I want to count each customer only once on the specific date (31.12.13). One thing that complicates it a lot is that some customers has term period some other date than 31st each year so that I cannot use 31st as a sorting element. Customer 2 has for example a term period 14. march each year, but it is still active 31st each year. Customer 3 has term period in february. This customer was active in february 2013, but inactive december 2013. I will therefor not count customer 3 as of 31st of December 2013.
I have attached a sample data.

Anyone?