Count milliseconds in Actionscript 3

In ActionScript 3 I would like to count the milliseconds of the difference between pressing two buttons.

Example:

boton1.onClick{
  $inicio = now (PHP);
}

boton2.onClick{
  $fin = now (PHP);
}

$milisegundos = $fin - $inicio;

To show the result I can use:

• echo $milisegundos;
• salida.Text = milisegundos;