code: private static boolean isValidDateTime(String lex) {
          try {
              DateUtility.parseDateStrict(lex);
              if (logger.isTraceEnabled()) {
                  logger.trace("Validated dateTime: " + lex);
              }
              return true;
          } catch (ParseException e) {
              logger.warn("Not a valid dateTime: " + lex);
              return false;
          }
      }
docs: Tells whether the given string is a valid lexical representation of a
  dateTime value. Passing this test will ensure successful indexing later.