code: function (number, digits) {
      if (typeof number !== "number") { throw "not a number: " + number; }
      var s = number.toString();
      if (number.length > digits) {
        return number.substr(number.length - digits, number.length);
      }
      while (s.length < digits) {
        s = '0' + s;
      }
      return s;
    }
docs: Format a number to the length = digits. For ex:  `_fixWidth(2, 2) = '02'`  `_fixWidth(1998, 2) = '98'`  This is used to pad numbers in converting date to string in ISO standard.