Thanks for your answer. I am sorry my worksheet "All" is confusing. I would like to have :
the number of purchases over the last 4 weeks / the number of distinct customers over the last 4 weeks
= what is in my worksheet "Nb Purchase" / what is in my worksheet "Nb Customer" :
Count distinct across moving period is bit tricky.
To achieve this, you need to duplicate data set x 4.
Edit data source => Union.
Now you got this table.
Using "Date2", calculation is straight forward.
it works perfectly, thank you very much !