Thanks for your input but unfortunately it isn´t a solution for me.
Let’s assume you have all months of 2013 and 2014. If you click on e.g. Jun and Jul 2014 then your data sets will be decrease on your selection. What I want is to show the sum of Jun and Jul 2014 and the value before my selected date (May 2014).
What I currently have or what you suggested works fine if I only select single dates and no intervals (Jun AND Jul 2014). Because if you click on more than one date your data sets are only what you selected (Jun and Jul) so the formula will only consider Jun and Jul 2014 but not the entire data sets in my application.
For that reason I need a formula which identify the youngest selected date (Jun 2014) and then use the entire data sets to get the value before the youngest date.