Earliest Date

= #date(Date.Year(List.Min(Source[OrderDate])),1,1)

Latest Date

= #date(Date.Year(List.Max(Source[RequiredDate])),12,31)

(Dynamic) Calendar
let
    Source = {Number.From(pmEarliestDate)..Number.From(pmLatestDate)},
    #"Converted to Table" = Table.FromList(Source, Splitter.SplitByNothing(), null, null, ExtraValues.Error),
    #"Renamed Columns" = Table.RenameColumns(#"Converted to Table",{{"Column1", "Date"}}),
    #"Changed Type" = Table.TransformColumnTypes(#"Renamed Columns",{{"Date", type date}})
in
    #"Changed Type"