code: @Nullable
    public static String getAsStringW3C (@Nullable final ZonedDateTime aDateTime)
    {
      if (aDateTime == null)
        return null;
      final DateTimeFormatter aFormatter = PDTFormatter.getForPattern (FORMAT_W3C, LOCALE_TO_USE);
      return aFormatter.format (aDateTime);
    }
docs: create a W3C Date Time representation of a date time using UTC date time
  zone.
  
  @param aDateTime
  Date to print. May not be <code>null</code>.
  @return the W3C Date Time represented by the given Date.