subunit-ls time handling if time not first instruction broken

test: canonical.testing.layers.FunctionalLayer:tearDown
time: 2011-05-20 16:37:05.820858Z

will cause

Traceback (most recent call last):
  File "/usr/bin/subunit-ls", line 39, in <module>
    test.run(result)
  File "/home/robertc/source/unittest/subunit/working/python/subunit/__init__.py", line 1151, in run
    protocol.lostConnection()
  File "/home/robertc/source/unittest/subunit/working/python/subunit/__init__.py", line 558, in lostConnection
    self._state.lostConnection()
  File "/home/robertc/source/unittest/subunit/working/python/subunit/__init__.py", line 354, in lostConnection
    self.parser._lostConnectionInTest(_u(''))
  File "/home/robertc/source/unittest/subunit/working/python/subunit/__init__.py", line 554, in _lostConnectionInTest
    self.client.stopTest(self._current_test)
  File "/usr/lib/python2.7/dist-packages/testtools/testresult/real.py", line 555, in stopTest
    return self.decorated.stopTest(test)
  File "/home/robertc/source/unittest/subunit/working/python/subunit/test_results.py", line 471, in stopTest
    test_duration = self._time() - self._start_time
TypeError: unsupported operand type(s) for -: 'datetime.datetime' and 'int'