'Provides the timestamp of the newest durable file found in the object
directory.
:return: A Timestamp instance, or None if no durable 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._ondisk_info.get('durable_frag_set'):
        return self._ondisk_info['durable_frag_set'][0]['timestamp']
    return None
