code: private static long getOldestIndexDate(CmsObject cms) {
  
          final File path = new File(getSolrSpellcheckRfsPath());
          final File[] directories = path.listFiles(SPELLCHECKING_DIRECTORY_NAME_FILTER);
  
          // Initialize with the greatest value a long type can hold
          long oldestIndexDate = Long.MAX_VALUE;
  
          for (final File dir : directories) {
              long date = dir.lastModified();
              if (date < oldestIndexDate) {
                  oldestIndexDate = date;
              }
          }
  
          // If no file(s) have been found oldestIndexDate is still holding
          // Long.MAX_VALUE. In that case return Long.MIN_VALUE to ensure
          // that no indexing operation takes place.
          if (Long.MAX_VALUE == oldestIndexDate) {
              LOG.warn("It appears that no spellcheck indices have been found in " + getSolrSpellcheckRfsPath() + ". ");
              return Long.MIN_VALUE;
          }
  
          return oldestIndexDate;
      }
docs: Returns the timestamp of the index whose index-built operation lies the
  furthest back in the past.<p>
  
  @param cms the OpenCms instance.
  @return timestamp as type long.