Python 3.3.5 (v3.3.5:62cf4e77f785, Mar 9 2014, 10:35:05) [MSC v.1600 64 bit (AMD64)] on win32
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>>> from datetime import *
>>> E = datetime(1970,1,1,tzinfo=timezone.utc)
>>> s = -1/2**7
>>> datetime.fromtimestamp(s, timezone.utc)
datetime.datetime(1969, 12, 31, 23, 59, 59, 992187, tzinfo=datetime.timezone.utc)
>>> E + timedelta(seconds=s)
datetime.datetime(1969, 12, 31, 23, 59, 59, 992187, tzinfo=datetime.timezone.utc)
FWIW Windows 10.