Custom Column Conundrum

Something like this might work.

Go to Transform Data and paste this into the advanced editor of a blank query...

let
    Source = Table.FromRows(
        Json.Document(
            Binary.Decompress(
                Binary.FromText(
                    "i45WMjDUNzDRNzIwtFDSUTKyQOIYAjGKLEjAsaAIpA4ioGsCJEBY18xAKVaHoGFG1DTMmJqGmVDTMFPiDYsFAA==",
                    BinaryEncoding.Base64
                ),
                Compression.Deflate
            )
        ),
        let
            _t = ((type nullable text) meta [Serialized.Text = true])
        in
            type table [
                #"Period Start Date" = _t,
                #"Period End Date" = _t,
                Index = _t,
                Date = _t,
                Period = _t,
                Month = _t,
                Year = _t,
                CurYearOffset = _t,
                MonthNum = _t,
                PeriodOffset = _t
            ]
    ),
    ChangeType = Table.TransformColumnTypes(
        Source,
        {
            {"Period Start Date", type date},
            {"Period End Date", type date},
            {"Date", type date},
            {"Index", Int64.Type},
            {"Period", Int64.Type},
            {"Year", Int64.Type},
            {"CurYearOffset", Int64.Type},
            {"MonthNum", Int64.Type},
            {"PeriodOffset", Int64.Type}
        }
    ),
    AddDaysInPeriod = Table.AddColumn(
        ChangeType,
        "DaysInPeriod",
        each Number.RoundTowardZero(Duration.TotalDays([Period End Date] - [Period Start Date])) + 1,
        Int64.Type
    ),
    AddNewPeriodOffset = Table.AddColumn(
        AddDaysInPeriod,
        "NewPeriodOffset",
        each
            Number.RoundTowardZero(
                Duration.TotalDays([Period Start Date] - DateTime.Date(DateTime.FixedLocalNow())) / [DaysInPeriod]
            ),
        Int64.Type
    )
in
    AddNewPeriodOffset