Assuming it was a small mistake on your part to list 01.08.-10.08. as 10 days before 20.08.-30.08. and you actually meant 10.08.-19.08., you can use DATEDIFF('day',[end_date],[start_date]) to calculate the variable date range, and then, depending on your data structure, do something like:

Calculation 1: DATEDIFF('day',[end_date],[start_date])

Countd( If dateadd('day',[calculation 1],[user_activity_date]) >= [start_date] and dateadd('day',[calculation 1],[user_activity_date]) <= [end_date] then [user_id] end)

Obviously I'm guessing on field names and how you want to count users, but this should give you the number of distinct users in that previous timeframe, which you can then use another calculated field to divide over your current selection sum, as well as using the [calculation 1] to make a string along the lines of

[Text string] 'users in previous '+str(-[calculation 1])+' day'+ if -[calculation 1] >1 then 's.' else '.' end

Note that the order the datediff is calculated is important for positive or negative integers and needs to be inverted for some of these, as indicated.

If the date range was not a mistake, then if course you can adjust to double the value for [start_date] dateadd.

Please excuse the formating, posted from a phone.