Calculate the sum of turnover on client account by date of loan

atasovski 

As variant

let
    getTurnover = Table.Group(
        Excel.CurrentWorkbook(){[Name="Tabelle1"]}[Content],
        {"Client ID", "LoanDate"},
        { {"Turnover",
            each [
                tbl = _,
                loanDate = tbl{0}[LoanDate],
                filter = Table.SelectRows( tbl,
                    (q) => q[TurnoverDate] <= loanDate and
                           q[TurnoverDate] >= Date.AddDays(loanDate, -30)),
                t = List.Sum( filter[TurnoverAmount] )
            ][t], Currency.Type
        }  } )
in
    getTurnover