Transform a table using Power Query

let
    Source = Excel.CurrentWorkbook(){[Name="Tabelle1"]}[Content],
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Name", type text}, {"Start", type date}, {"End", type date}}),
    #"Added Custom" = Table.AddColumn(#"Changed Type", "Benutzerdefiniert", each List.Distinct(List.Transform({Number.From([Start])..Number.From([End])}, each Date.StartOfMonth(Date.From(_))))),
    #"Expanded {0}" = Table.ExpandListColumn(#"Added Custom", "Benutzerdefiniert"),
    #"Renamed Columns" = Table.RenameColumns(#"Expanded {0}",{{"Benutzerdefiniert", "Period"}}),
    #"Removed Columns" = Table.RemoveColumns(#"Renamed Columns",{"Start", "End"})
in
    #"Removed Columns"

This code returns the intended result in my sample file. For column Period in the green result table i've selected costum format MMM YYYY.