docs: function getDayPeriods(localeData, dayPeriodsList) {
    const dayPeriods = localeData.main(`dates/calendars/gregorian/dayPeriods`);
    const result = {};
    // cleaning up unused keys
    Object.keys(dayPeriods).forEach(key1 => {          // format / stand-alone
      result[key1] = {};
      Object.keys(dayPeriods[key1]).forEach(key2 => {  // narrow / abbreviated / wide
        result[key1][key2] = {};
        Object.keys(dayPeriods[key1][key2]).forEach(key3 => {
          if (dayPeriodsList.indexOf(key3) !== -1) {
            result[key1][key2][key3] = dayPeriods[key1][key2][key3];
          }
        });
      });
    });
  
    return result;
  }