code: function index_date(timestamp, interval) {
      function double_digitize(str) {
          return (str.length === 1) ? ('0' + str) : str;
      }
  
      var year = timestamp.getUTCFullYear();
      var month = (timestamp.getUTCMonth() + 1).toString();
  
      switch (interval) {
          case 'day':
              var day = timestamp.getUTCDate().toString();
              return [year, double_digitize(month), double_digitize(day)].join('.');
  
          case 'week':
              var week = current_week_number(timestamp);
              return [year, double_digitize(week)].join('.');
  
          case 'month':
              return [year, double_digitize(month)].join('.');
  
          case 'year':
              return year;
  
          case 'none':
              return '';
  
          default:
              throw new Error('invalid interval: ' + interval + '; accepted intervals are "day", "week", "month", "year", and "none"');
      }
  }
docs: YYYY.MM.dd