docs: function TimeTraveler(settings) {
  
    // coerce the +starts_at+ value into a moment
    var starts_at_moment = moment(settings.starts_at);
  
    // initialize new TimeTraveler object attributes
    this.starts_at = starts_at_moment;
    this.steps = settings.steps;
    this.time_units = settings.time_units;
    this.time_scale = settings.time_scale || 1;
    this.current = { time: starts_at_moment, step: 0 };
  }