You can't solve this with set analysis expressions. Those are calculated once for the entire chart, not per row. If you have Values for every day and the dates are ordered like in your example you can use above(sum(Value),7) as the expression to get the sum(Value) of 7 rows above, which in this case is also 7 days ago.
That's not possibly with set analysis expressions. The set is calculated for the entire chart/table, not per row. You'll have to deal with it in the load script. It's fairly easy. See attached example.