code: protected long computeContainerPageModificationDate(CmsResource containerPage) throws CmsException {
  
          CmsRelationFilter filter = CmsRelationFilter.relationsFromStructureId(
              containerPage.getStructureId()).filterType(CmsRelationType.XML_STRONG);
          List<CmsRelation> relations = m_guestCms.readRelations(filter);
          long result = containerPage.getDateLastModified();
          for (CmsRelation relation : relations) {
              try {
                  CmsResource target = relation.getTarget(
                      m_guestCms,
                      CmsResourceFilter.DEFAULT_FILES.addRequireVisible());
                  long targetDate = target.getDateLastModified();
                  if (targetDate > result) {
                      result = targetDate;
                  }
              } catch (CmsException e) {
                  LOG.warn(
                      "Could not get relation target for relation "
                          + relation.toString()
                          + " | "
                          + e.getLocalizedMessage(),
                      e);
              }
          }
  
          return result;
      }
docs: Computes the container the container page modification date from its referenced contents.<p>
  
  @param containerPage the container page
  
  @return the computed modification date
  
  @throws CmsException if something goes wrong