code: @Nonnull
    public Cell addCell (@Nullable final ZonedDateTime aValue)
    {
      if (aValue == null)
        return addCell ();
      return addCell (GregorianCalendar.from (aValue));
    }
docs: Added a new cell as date/time.
  <p>
  Important: don't forget to call {@link #addCellStyle(ExcelStyle)} with
  something like <code>new ExcelStyle ().setDataFormat ("dd.mm.yyyy");</code>
  after a date/time cell!
  <p>
  Important: Excel cannot correctly handle dates/times before
  {@link CExcel#EXCEL_MINIMUM_DATE 1900-01-01}
  
  @param aValue
  The value to be set. May be <code>null</code>.
  @return A new cell in the current row of the current sheet with the passed
  value