code: public void setDateHeader (@Nonnull @Nonempty final String sName, final long nMillis)
    {
      setDateHeader (sName, PDTFactory.createZonedDateTime (nMillis));
    }
docs: Set the passed header as a date header.
  
  @param sName
  Header name. May neither be <code>null</code> nor empty.
  @param nMillis
  The milliseconds to set as a date.