module Calendar =

  open System

  open System.Globalization

​

  let private culture = CultureInfo.InvariantCulture

​

  let private months =

    let mfi = culture.DateTimeFormat

    [1..12]

      |> List.map (fun n -> n, mfi.GetMonthName n)

      |> List.mapi (fun i m -> pown 2 i, m)

      |> Map.ofList

​

  let extractMonths x =

    Map.foldBack

      (fun k v ls ->

        if x &&& k = k then

          v::ls

        else

          ls)

      months

      []

​

  let firstDay year month =

    DateTime(year, month, 1)

​

  let lastDay year month =

    let lastDay = culture.Calendar.GetDaysInMonth(year, month)

    DateTime(year, month, lastDay)

​

let findConsecutives xs =

  let collect x = function

    | (y::xs)::xss when y - x <= 1 -> (x::y::xs)::xss

    | xss -> [x]::xss

  List.foldBack collect xs []

​

type Range = { Start: System.DateTime; Ende: System.DateTime }

​

let extractRange year months =

  months

    |> List.map fst

    |> findConsecutives

    |> List.map (fun ls ->

      { Start = List.head ls |> Calendar.firstDay year

        Ende = Seq.last ls |> Calendar.lastDay year })

​

(******)

​

Calendar.extractMonths 910

  |> extractRange 2016

  |> printfn "%A"

​