code: function CronExpression (fields, options) {
    this._options = options;
    this._utc = options.utc || false;
    this._tz = this._utc ? 'UTC' : options.tz;
    this._currentDate = new CronDate(options.currentDate, this._tz);
    this._startDate = options.startDate ? new CronDate(options.startDate, this._tz) : null;
    this._endDate = options.endDate ? new CronDate(options.endDate, this._tz) : null;
    this._fields = fields;
    this._isIterator = options.iterator || false;
    this._hasIterated = false;
    this._nthDayOfWeek = options.nthDayOfWeek || 0;
  }
docs: Construct a new expression parser
  
  Options:
  currentDate: iterator start date
  endDate: iterator end date
  
  @constructor
  @private
  @param {Object} fields  Expression fields parsed values
  @param {Object} options Parser options