static parseSpecifier(s) {
    if (s) {
      const r = s.match(/^utc(?:([+-]\d{1,2})(?::(\d{2}))?)?$/i);
      if (r) {
        return new FixedOffsetZone(signedOffset(r[1], r[2]));
      }
    }
    return null;
  }
Get an instance of FixedOffsetZone from a UTC offset string, like "UTC+6"
@param {string} s - The offset string to parse
@example FixedOffsetZone.parseSpecifier("UTC+6")
@example FixedOffsetZone.parseSpecifier("UTC+06")
@example FixedOffsetZone.parseSpecifier("UTC-6:00")
@return {FixedOffsetZone}