code: public void addDateHeader (@Nonnull @Nonempty final String sName, @Nonnull final ZonedDateTime aDT)
    {
      _addHeader (sName, getDateTimeAsString (aDT));
    }
docs: Add the passed header as a date header.
  
  @param sName
  Header name. May neither be <code>null</code> nor empty.
  @param aDT
  The DateTime to set as a date. May not be <code>null</code>.