Calculate evolution between two dates using a slicer

Hello, 

I have an issue calculating the evolution in a period on wich I can change the beginning and ending dates.

I can't find the beginning date, so I can't calculate the value on this date, I tried : 

StartDate = CALCULATE(min('NPS'[DATE]),ALLSELECTED('NPS'[DATE]))

but it still gives me the end date, I don't understand why...