type msg =
    | Fetch of AsyncReplyChannel<int>

let counter =
    MailboxProcessor.Start(fun inbox ->
        let rec loop (lastRequestTime: DateTime) lastResult =
            async { let! msg = inbox.Receive()
                    match msg with
                    | Fetch(replyChannel) ->
                        match DateTime.Now.Subtract(lastRequestTime).TotalSeconds < 5.0 with
                        | true ->
                            replyChannel.Reply(lastResult)
                            return! loop lastRequestTime lastResult
                        | false ->
                            let newResult = lastResult + 1
                            replyChannel.Reply(newResult)
                            return! loop DateTime.Now newResult }
        loop (new DateTime(2015, 01, 01)) 0)