Unpivoting nested column headers

Anonymous 

Please try this

See the attached file as well

You can follow the steps from the Query Editor

All of this was user Interface driven (No manual coding required)

let
    Source = Excel.CurrentWorkbook(){[Name="Table1"]}[Content],
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Column1", type text}, {"Column2", type text}, {"Column3", type any}, {"Column4", type any}, {"Column5", type any}, {"Column6", type any}, {"Column7", type any}, {"Column8", type any}, {"Column9", type any}, {"Column10", type any}, {"Column11", type any}}),
    #"Transposed Table" = Table.Transpose(#"Changed Type"),
    #"Merged Columns" = Table.CombineColumns(Table.TransformColumnTypes(#"Transposed Table", {{"Column1", type text}, {"Column2", type text}}, "en-US"),{"Column1", "Column2"},Combiner.CombineTextByDelimiter(",", QuoteStyle.None),"Merged"),
    #"Transposed Table1" = Table.Transpose(#"Merged Columns"),
    #"Promoted Headers" = Table.PromoteHeaders(#"Transposed Table1", [PromoteAllScalars=true]),
    #"Changed Type1" = Table.TransformColumnTypes(#"Promoted Headers",{{",", type text}, {"Branch,Month", type text}, {"1000,10/1/2018 12:00:00 AM", type number}, {"1000,11/1/2018 12:00:00 AM", type number}, {"1000,12/1/2018 12:00:00 AM", type number}, {"1001,10/1/2018 12:00:00 AM", type number}, {"1001,11/1/2018 12:00:00 AM", type number}, {"1001,12/1/2018 12:00:00 AM", type number}, {"1002,10/1/2018 12:00:00 AM", type number}, {"1002,11/1/2018 12:00:00 AM", type number}, {"1002,12/1/2018 12:00:00 AM", type number}}),
    #"Unpivoted Columns" = Table.UnpivotOtherColumns(#"Changed Type1", {",", "Branch,Month"}, "Attribute", "Value"),
    #"Split Column by Delimiter" = Table.SplitColumn(#"Unpivoted Columns", "Attribute", Splitter.SplitTextByDelimiter(",", QuoteStyle.Csv), {"Attribute.1", "Attribute.2"}),
    #"Changed Type2" = Table.TransformColumnTypes(#"Split Column by Delimiter",{{"Attribute.1", Int64.Type}, {"Attribute.2", type datetime}}),
    #"Renamed Columns" = Table.RenameColumns(#"Changed Type2",{{",", "Account"}, {"Branch,Month", "Sub Account"}, {"Attribute.1", "Branch"}, {"Attribute.2", "Month"}})
in
    #"Renamed Columns"