Windows form application of stopwatch in 30 lines of code in F#

gistfile1.fs

open System
open System.Drawing
open System.Windows.Forms

let mutable startTime = new DateTime()

let timerLabel = new Label(Text="0",Location=Point(0,0))
let startButton = new Button(Text="Start",Location=Point(0,24))
let stopButton = new Button(Text="Stop",Location=Point(0,48))
let clearButton = new Button(Text="Clear",Location=Point(0,72))

let timer = new Timer(Interval=20)
timer.Tick.Add (fun _ -> 
  timerLabel.Text <- string <| DateTime.Now - startTime)
  
startButton.Click.Add begin fun _ -> 
  if String.IsNullOrEmpty timerLabel.Text then
    startTime <- DateTime.Now
  else
    startTime <- DateTime.Now - TimeSpan.Parse timerLabel.Text
  timer.Start()
  end

stopButton.Click.Add (fun _ ->timer.Stop())
clearButton.Click.Add (fun _ ->
  timer.Stop()
  timerLabel.Text <- "")

let form = new Form(Text="Stop Watch")
form.Controls.AddRange [|timerLabel;startButton;stopButton;clearButton|]

Application.Run(form)