'Provides the timestamp of the newest data file found in the object
directory.
:return: A Timestamp instance, or None if no data file was found.
:raises DiskFileNotOpen: if the open() method has not been previously
called on this instance.'

@property
def durable_timestamp(self):

    if (self._ondisk_info is None):
        raise DiskFileNotOpen()
    if self._datafile_metadata:
        return Timestamp(self._datafile_metadata.get('X-Timestamp'))
    return None
