let
    Source    = Excel.CurrentWorkbook(){[Name="Таблица1_2"]}[Content],
    NewPeriod = Table.AddColumn(Source, "Пользовательская", each let 
                    period = List.Buffer(Text.Split([period],", ")),
                    num    = List.Buffer(List.Transform(period,each Date.Month(Date.From(_&"1")))),
                    sorted = List.Sort(
                                 List.Generate(
                                     ()=> [i=0,m=num{i}?,y=1],
                                     each [m]<>null,
                                     each [i=[i]+1,m=num{i}?,y=[y]-(if m>[m]+1 then 1 else 0)], 
                                     each period{[i]} meta [d=#date([y],[m],1)]
                                 ),
                                 each Value.Metadata(_)[d]
                             ),
                    result  = Text.Combine(List.RemoveRange(sorted,1,List.Count(num)-2),"-")
                in result)
in
    NewPeriod