code: public void lockToStashCreatedAt(Date creationTime)
              throws StashNotAvailableException {
          String stashDirectory = StashUtil.getStashDirectoryForCreationTime(creationTime);
          // The following call will raise an AmazonS3Exception if the file cannot be read
          try (S3Object s3Object = _s3.getObject(_bucket, String.format("%s/%s/%s", _rootPath, stashDirectory, StashUtil.SUCCESS_FILE))) {
              _lockedLatest = String.format("%s/%s", _rootPath, stashDirectory);
          } catch (AmazonS3Exception e) {
              if (e.getStatusCode() == 404 ||
                      // The following conditions indicate the file has already been moved to Glacier
                      (e.getStatusCode() == 403 && "InvalidObjectState".equals(e.getErrorCode()))) {
                  throw new StashNotAvailableException();
              }
              throw e;
          } catch (IOException e) {
              // Shouldn't happen since the file is never actually read
          }
      }
docs: This method is like {@link #lockToLatest()} except that the caller requests a specific Stash time.
  @throws StashNotAvailableException Thrown if no Stash is available for the given time