code: function markActivity(activityId) {
    var activity = getActivity(activityId);
    var timeNow = Date.now();
    activity.timestamps.push(timeNow);
    return timeNow;
  }
docs: Mark a timestamp in an activity. Adds it to the array of timestamps.
  @param {number} activityId Activity ID.
  @return {number} The timestamp value added.