code: function canonicalizeTimeZoneID(tzID) {
    // Skip undefined zones.
    if (IS_UNDEFINED(tzID)) {
      return tzID;
    }
  
    // Special case handling (UTC, GMT).
    var upperID = %StringToUpperCase(tzID);
    if (upperID === 'UTC' || upperID === 'GMT' ||
        upperID === 'ETC/UTC' || upperID === 'ETC/GMT') {
      return 'UTC';
    }
  
    // TODO(jshin): Add support for Etc/GMT[+-]([1-9]|1[0-2])
  
    // We expect only _, '-' and / beside ASCII letters.
    // All inputs should conform to Area/Location(/Location)* from now on.
    var match = InternalRegExpMatch(GetTimezoneNameCheckRE(), tzID);
    if (IS_NULL(match)) throw MakeRangeError(kExpectedTimezoneID, tzID);
  
    var result = toTitleCaseTimezoneLocation(match[1]) + '/' +
                 toTitleCaseTimezoneLocation(match[2]);
  
    if (!IS_UNDEFINED(match[3]) && 3 < match.length) {
      var locations = %_Call(StringSplit, match[3], '/');
      // The 1st element is empty. Starts with i=1.
      for (var i = 1; i < locations.length; i++) {
        result = result + '/' + toTitleCaseTimezoneLocation(locations[i]);
      }
    }
  
    return result;
  }
docs: Returns canonical Area/Location(/Location) name, or throws an exception
  if the zone name is invalid IANA name.