Partial Traditional Stopwatch

WARNING: This one might go down in flames or be subject to recall!

class Timer {
  long timeStarted;
  long timeStopped;
  Timer() {
    this.reset();
  }
  void reset() {
    this.timeStarted = -1;
    this.timeStopped = -1;
  }
  void start() {
    this.timeStarted = System.currentTimeMillis();
  }
  void stop() {
    this.timeStopped = System.currentTimeMillis();
  }
  boolean isStarted() {
    return this.timeStarted != -1;
  }
  boolean isStopped() {
    return this.timeStopped != -1;
  }
  boolean isRunning() {
    return this.isStarted() && !this.isStopped();
  }
  long elapsedTimeInMs() {
    if (this.timeStarted != -1) {
      if (this.timeStopped != -1) {
        return this.timeStopped - this.timeStarted;
      }
      else {
        return System.currentTimeMillis() - this.timeStarted;
      }
    }
    else {
      return -1;
    }
  }
}

/** FROM: https://en.wikipedia.org/wiki/Stopwatch
 *
 * A stopwatch is a handheld timepiece designed to measure
 * the amount of time elapsed from a particular time when it
 * is activated to the time when the piece is deactivated.
 * 
 * The timing functions are traditionally controlled by two
 * buttons on the case. Pressing the top button starts the
 * timer running, and pressing the button a second time stops
 * it, leaving the elapsed time displayed. A press of the
 * second button then resets the stopwatch to zero. The second
 * button is also used to record split times or lap times.
 * When the split time button is pressed while the watch is
 * running, the display freezes, allowing the elapsed time to
 * that point to be read, but the watch mechanism continues
 * running to record total elapsed time. Pressing the split
 * button a second time allows the watch to resume display of
 * total time.
 */
class Stopwatch {
  Timer timer;
  Stopwatch() {
    this.timer = new Timer();
  }
  /** topButtonPress:
   *    Starts timer if timer is not started.
   *    Stops timer if timer is running.
   *    Returns:
   *      0 if button press causes timer to start.
   *      Elapsed time in ms otherwise.
   */
  long topButtonPress() {
    // MISSING CODE!
    return -666;
  }
  /** sideButtonPress:
   *    Resets timer if timer is not running.
   *    Returns:
   *      -1 if button press causes timer to reset.
   *      Elapsed time in ms otherwise.
   */
  long sideButtonPress() {
    // MISSING CODE!
    return -666;
  }
}

class TraditionalStopwatch {
  public static void main(String[] args) {
    Stopwatch s1 = new Stopwatch();
    s1.topButtonPress();
    // Long.MAX_VALUE is 9,223,372,036,854,775,807
    for (long i=0; i < 1000000000; i++);
    long elapsedTime = s1.topButtonPress();
    System.out.println(elapsedTime);
  }
}

function Timer() {
  this.reset();
}
Timer.prototype = {
  reset: function() {
    this.timeStarted = null;
    this.timeStopped = null;
  },
  start: function() {
    this.timeStarted = new Date();
  },
  stop: function() {
    this.timeStopped = new Date();
  },
  isStarted: function() {
    return this.timeStarted != null;
  },
  isStopped: function() {
    return this.timeStopped != null;
  },
  isRunning: function() {
    return this.isStarted() && !this.isStopped();
  },
  elapsedTimeInMs: function() {
    if (this.timeStarted != null) {
      if (this.timeStopped != null) {
        return this.timeStopped - this.timeStarted;
      }
      else {
        return new Date() - this.timeStarted;
      }
    }
    else {
      return undefined;
    }
  }
};

/** FROM: https://en.wikipedia.org/wiki/Stopwatch
 *
 * A stopwatch is a handheld timepiece designed to measure
 * the amount of time elapsed from a particular time when it
 * is activated to the time when the piece is deactivated.
 * 
 * The timing functions are traditionally controlled by two
 * buttons on the case. Pressing the top button starts the
 * timer running, and pressing the button a second time stops
 * it, leaving the elapsed time displayed. A press of the
 * second button then resets the stopwatch to zero. The second
 * button is also used to record split times or lap times.
 * When the split time button is pressed while the watch is
 * running, the display freezes, allowing the elapsed time to
 * that point to be read, but the watch mechanism continues
 * running to record total elapsed time. Pressing the split
 * button a second time allows the watch to resume display of
 * total time.
 */
function Stopwatch() {
  this.timer = new Timer();
}
Stopwatch.prototype = {
  /** topButtonPress:
   *    Starts timer if timer is not started.
   *    Stops timer if timer is running.
   *    Returns:
   *      0 if button press causes timer to start.
   *      Elapsed time in ms otherwise.
   */
  topButtonPress: function() {
    // MISSING CODE!
  },
  /** sideButtonPress:
   *    Resets timer if timer is not running.
   *    Returns:
   *      -1 if button press causes timer to reset.
   *      Elapsed time in ms otherwise.
   */
  sideButtonPress: function() {
    // MISSING CODE!
  },
}

var s1 = new Stopwatch();
s1.topButtonPress();
// Number.MAX_SAFE_INTEGER in JavaScript is 2^53 - 1
for (var i=0; i <= 1000000000; i++);
var elapsedTime = s1.topButtonPress();
alert(elapsedTime);