Another spilled array that I can't solve - Aggregating monthly into time periods

James_Buist 

An interesting set of data. I though of unpivoting it first before proceeding but found it more straightforward to use MAKEARRAY. 

=LET(
    i, ROWS(data),
    j, COLUMNS(data),
    GetTotals, LAMBDA(r, c,
        LET(
            current_row, CHOOSEROWS(data, r),
            each_period, INDEX(periods, r),
            start, IFNA(XMATCH(TRUE, current_row <> 0, 1), 0) + each_period - 1,
            k, ROUNDUP(SUM(N(current_row <> 0)) / each_period, 0),
            seq₁, SEQUENCE(k, , start, each_period),
            seq₂, SEQUENCE(each_period, , c, -1),
            total, SUM(INDEX(data, r, seq₂)),
            check, XOR(seq₁ = c),
            result, IF(check, total, 0),
            IFERROR(result, 0)
        )
    ),
    MAKEARRAY(i, j, GetTotals)
)