YYYYDDD to gregorian date

let
  Source = Table.FromRows(
    Json.Document(
      Binary.Decompress(
        Binary.FromText("i45WMjIwMjIwMFSK1YGwDc1MlWJjAQ==", BinaryEncoding.Base64), 
        Compression.Deflate
      )
    ), 
    let
      _t = ((type nullable text) meta [Serialized.Text = true])
    in
      type table [Column1 = _t]
  ), 
  #"Added Custom" = Table.AddColumn(
    Source, 
    "Custom", 
    each Date.ToText(
      Date.AddDays(
        Date.From(Text.Start([Column1], 4) & "-01-01"), 
        Int32.From(Text.End([Column1], 3)) - 1
      ), 
      "yyyy-mm-dd"
    )
  )
in
  #"Added Custom"

Instead of converting your date column, you can convert your RangeStart and RangeEnd parameters, and then use those values in your filter step. It will still fold. Here is an example.

let
    RSasYYYYDDD = 1000 * Date.Year(RangeStart) + Date.Day(RangeStart),
    REasYYYYDDD = 1000 * Date.Year(RangeEnd) + Date.Day(RangeEnd),
    Source = Sql.Database("localhost", "Flights", [CreateNavigationProperties=false]),
    dbo_Date = Source{[Schema="dbo",Item="Date"]}[Data],
    #"Filtered Rows" = Table.SelectRows(dbo_Date, each [Date] >= RSasYYYYDDD and [Date] < REasYYYYDDD)
in
    #"Filtered Rows"

Pat