keepind dates as column headers

Hello Anonymous 

in your case your headers in Excel are stored as date (01/01/2020) but visualized in another format like Apr-20. As Power BI is reading the real data in the cells, not the visualisation. So, or you transform your data in Excel like real text or you apply something like that

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjDRNzDUNzIwMlDSUTIwReaYwTmxOtFKhkAhIyA2VoqNBQA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Column1 = _t, Column2 = _t, Column3 = _t]),
    #"Added Index" = Table.AddIndexColumn(Source, "Index", 0, 1),
    TransformFirstRow = Table.FromRecords( Table.TransformRows
    (
        #"Added Index",
        (rec)=> if rec[Index]=0 then Record.FromTable(Table.TransformColumns(Record.ToTable(rec), {{"Value", each Date.ToText(Date.From(_,"en-US"),"MMM-YY","en-US")}})) else rec
    )),
    #"Removed Columns" = Table.RemoveColumns(TransformFirstRow,{"Index"}),
    PromoteHeader = Table.PromoteHeaders(#"Removed Columns")
in
    PromoteHeader

Excel-import

after transformation

I've simulate the source-step as if you would access to your Excel-sheet

Copy paste this code to the advanced editor in a new blank query to see how the solution works.

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Have fun

Jimmy