Subtract Columns based on Position during Transform Data Stage

Hi zia_ward ,

You can try this query:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("VdBBCoAwDETRu7gWmiZNW88i3v8aimDnSzef8shiznNz81qet+0rU+32tYs4iIOESIBEiUWaSANpICmSIAnSRTpIBxkiA2SATJEJMkus/6MwU60r1bCdcTzDofqb9R3wum4=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"2021/1/1" = _t, #"2021/1/15" = _t, #"2021/1/20" = _t, #"2021/2/1" = _t, #"2021/2/15" = _t, #"2021/2/20" = _t, #"2021/3/1" = _t, #"2021/3/15" = _t, #"2021/3/30" = _t, #"2021/4/1" = _t, #"2021/4/15" = _t, #"2021/4/30" = _t, #"2021/5/1" = _t, #"2021/5/15" = _t, #"2021/5/30" = _t, #"2021/6/1" = _t, #"2021/6/15" = _t, #"2021/6/30" = _t, #"2021/7/1" = _t, #"2021/7/15" = _t, #"2021/7/30" = _t, #"2021/8/1" = _t, #"2021/8/15" = _t, #"2021/8/31" = _t, #"2021/9/1" = _t, #"2021/9/15" = _t, #"2021/9/30" = _t, #"2021/10/1" = _t, #"2021/10/15" = _t, #"2021/10/31" = _t, #"2021/11/15" = _t, #"2021/12/15" = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"2021/1/1", type date}, {"2021/1/15", type date}, {"2021/1/20", type date}, {"2021/2/1", type date}, {"2021/2/15", type date}, {"2021/2/20", type date}, {"2021/3/1", type date}, {"2021/3/15", type date}, {"2021/3/30", type date}, {"2021/4/1", type date}, {"2021/4/15", type date}, {"2021/4/30", type date}, {"2021/5/1", type date}, {"2021/5/15", type date}, {"2021/5/30", type date}, {"2021/6/1", type date}, {"2021/6/15", type date}, {"2021/6/30", type date}, {"2021/7/1", type date}, {"2021/7/15", type date}, {"2021/7/30", type date}, {"2021/8/1", type date}, {"2021/8/15", type date}, {"2021/8/31", type date}, {"2021/9/1", type date}, {"2021/9/15", type date}, {"2021/9/30", type date}, {"2021/10/1", type date}, {"2021/10/15", type date}, {"2021/10/31", type date}, {"2021/11/15", type date}, {"2021/12/15", type date}}),
    ColumnNames = Table.ColumnNames(#"Changed Type"),
    #"Added column" = 
        Table.AddColumn(
            #"Changed Type","Diff",each 
            let Last30 = List.LastN(ColumnNames,31) in 
                Number.From(Date.From(List.Last(Last30)) - Date.From(List.First(Last30))),
            type number
        )
in
    #"Added column"

Best Regards,
Community Support Team _ Yingjie Li
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