Difference in Months - Columns

Hello - this is how you can do it in Power Query. Basically you would add a column which contains a list of months from the start to the end date, expand the list to rows and pivot it to columns.

Script

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45W8srPyFMIzs0syVDSUTI0MjYBUqb6hvpGBkbGYBF9YygnNhYA", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Employee = _t, ID = _t, #"Start Date" = _t, #"End Date" = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Employee", type text}, {"ID", Int64.Type}, {"Start Date", type date}, {"End Date", type date}}),
    #"Added Custom" = Table.AddColumn(#"Changed Type", "Custom", each List.Transform ( List.Select ( List.Dates([Start Date], Number.From ( [End Date] ) - Number.From ( [Start Date] ),#duration(1,0,0,0)), each Date.Day ( _ ) = 1 ), each Date.ToText ( _, "MMMyy" ) ) ),
    #"Expanded Custom" = Table.ExpandListColumn(#"Added Custom", "Custom"),
    #"Removed Other Columns" = Table.SelectColumns(#"Expanded Custom",{"Employee", "ID", "Custom"}),
    #"Pivoted Column" = Table.Pivot(Table.TransformColumnTypes(#"Removed Other Columns", {{"Custom", type text}}, "en-US"), List.Distinct(Table.TransformColumnTypes(#"Removed Other Columns", {{"Custom", type text}}, "en-US")[Custom]), "Custom", "Employee", List.Count)
in
    #"Pivoted Column"

Result

FYI, when needing to show dates in columns instead of rows there is usually a better way of designing the solution, such as using a data table in the model.