Dynamic number of calculated columns (Project Online schedule dates)

iainh Here is a more refined approach in case there are duplicate publish dates, refer to Solution2

let
    Source = Data,
    Unpivot = 
        Table.UnpivotOtherColumns ( Source, { "Project #" }, "Publish#", "Published On" ),
    RemoveBlanks = 
        Table.SelectRows ( Unpivot, each [Published On] <> " " and [Published On] <> null ),
    ChangedType = 
        Table.TransformColumnTypes (
            RemoveBlanks,
            { { "Published On", type date }, { "Project #", Int64.Type } },
            "en-GB"
        ),
    GroupedRows = Table.Group (
        ChangedType,
        { "Project #" },
        {
            {
                "Transform",
                each
                    let
                        a = _,
                        b = a[Published On],
                        c = 
                            List.Accumulate (
                                b,
                                [ Dates = { List.First ( b ) }, vsPrevious = { 0 }, vsNext = { 0 } ],
                                ( s, c ) =>  
                                [
                                    Dates = s[Dates] & { c },
                                    vsPrevious = 
                                        s[vsPrevious] 
                                            & { Duration.Days ( c - List.Last ( s[Dates] ) ) },
                                    vsNext = 
                                        s[vsNext] 
                                            & { 
                                                Number.Abs ( 
                                                    Duration.Days ( List.Last ( s[Dates] ) - c ) 
                                                ) 
                                            }
                                ]
                            ),
                        d =
                            Record.TransformFields (
                                c,
                                {
                                    { "vsPrevious", List.Skip },
                                    { "vsNext", each List.Skip ( _ , 2) & {0} }
                                }
                            ),
                        e = { { "n - 1" } & d[vsPrevious] } & { { "n + 1" } & d[vsNext] },
                        f = Table.ToColumns ( Table.DemoteHeaders ( a ) ),
                        g = Table.PromoteHeaders ( Table.FromColumns ( f & e ) )
                    in
                        g
            }
        }
    ),
    CombineTables = 
        Table.Combine ( 
            GroupedRows[Transform], 
            type table [
                #"Project #" = Int64.Type,
                #"Publish#" = text, 
                Published On = date, 
                #"n - 1" = Int64.Type, 
                #"n + 1" = Int64.Type
            ] 
        )
in
    CombineTables

File attached below