Yes, since the timestamp is already in UTC, all that pytz is doing for you is a fancy way to add "+00:00" to the end of the string:

```python
>>> t = time.time()
>>> d=datetime.datetime.utcfromtimestamp(t)
>>> d
datetime.datetime(2014, 1, 3, 2, 25, 16, 197281)
>>> l = pytz.utc.localize(d)
>>> l
datetime.datetime(2014, 1, 3, 2, 25, 16, 197281, tzinfo=<UTC>)
>>> l.isoformat()
'2014-01-03T02:25:16.197281+00:00'
>>> d.isoformat()
'2014-01-03T02:25:16.197281'
>>> datetime.datetime.utcfromtimestamp(t).isoformat()[:-3]+'Z'
'2014-01-03T02:25:16.197Z'
```
So, I think that it's just simpler to skip pytz. Also, as mentioned in the comments, 1.3.2 has a bug where it won't accept timestamps with more than millisecond precision.