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# If you run the code below on Py30a3 you get the output shown at the end
import calendar, datetime, time
pastdate = datetime.datetime(1969, 12, 31)
print(pastdate)
timestamp = calendar.timegm(pastdate.utctimetuple())
print(timestamp)
try:
 pastdate_x = datetime.datetime.utcfromtimestamp(timestamp)
except ValueError as err:
 print("FAIL", err)
try:
 print(time.strftime("%Y-%m-%dT%H:%M:%S", time.gmtime(timestamp)))
except ValueError as err:
 print("FAIL", err)
r"""
Python 30a3
Windows output:
1969年12月31日 00:00:00
-86400
FAIL timestamp out of range for platform localtime()/gmtime() function
FAIL (22, 'Invalid argument')
Linux output:
1969年12月31日 00:00:00
-86400
1969年12月31日T00:00:00
"""
# What this appears to show is that you can't round-trip between
datetimes and timestamps on Windows for dates prior to 1970

It's most likely a platform limitation. Some platforms doen't support
negative time stamps.

If you doesn't care to the time zone (UTC), use:
>>> datetime.datetime(1970, 1, 1) + datetime.timedelta(seconds=-86400)
datetime.datetime(1969, 12, 31, 0, 0)
This code is portable on Windows, Linux but also works with 
IronPython:
http://www.codeplex.com/IronPython/WorkItem/View.aspx?WorkItemId=7269 

Indeed, the time module is documented as not handling timestamps before the epoch.