Convert microsecond timestamp to datetime in Python

Question

I'm pulling a cookie expiration date from Google Chrome. From what it looks like, Chrome is storing cookie expirations with a timestamp that uses 1601-01-01 00:00:00 UTC as the epoch. My current implementation is as follows:

stamp = int(result[3])
date = datetime.datetime.fromtimestamp(stamp / 10000000.0)
print date.year

However, this is producing the wrong date (off by about a year). What am I doing wrong here?

Answer 1

Another option, getting tzinfo from the standard library since Python 3.2 (for older Python versions you can get if from pytz):

>>> import pytz
>>> from datetime import datetime, timedelta, timezone
>>> epoch = datetime(1601, 1, 1, tzinfo=timezone.utc)
>>> cookie_microseconds_since_epoch = 13022344559000000
>>> cookie_datetime = epoch + timedelta(microseconds=cookie_microseconds_since_epoch)
>>> str(cookie_datetime)
'2013-08-29 13:55:59+00:00'

I assume that the difference to your expected value is the timezones offset.

Update:

As @J.F.Sebastian correctly points out, if you are using implicit UTC naive datetime objects, tzinfo is redundant and the above can be simplified to:

>>> from datetime import datetime, timedelta
>>> epoch = datetime(1601, 1, 1)
>>> cookie_microseconds_since_epoch = 13022344559000000
>>> cookie_datetime = epoch + timedelta(microseconds=cookie_microseconds_since_epoch)
>>> str(cookie_datetime)
'2013-08-30 13:55:59'

Answer 2

I'm not sure what data you're starting with but here is an example starting from an integer timestamp. Assumes the pytz module is present (which I recommend highly).

>>> import datetime, pytz
>>> x = datetime.datetime.fromtimestamp(0)
>>> x = x.replace(tzinfo=pytz.UTC)
>>> str(x)
'1970-01-01 00:00:00+00:00'
>>> d = datetime.timedelta(365 * (1970 - 1601))
>>> str(x - d)
'1601-03-31 00:00:00+00:00'
>>> d = datetime.timedelta(365 * (1970 - 1601) + 31 + 28 + 31 - 1)
>>> str(x - d)
'1601-01-01 00:00:00+00:00'
>>> str(d)
'134774 days, 0:00:00'

So there you have it. Conversion between a Jan 1 1601 epoch and a Jan 1 1970 epoch is 134774 days.

Why that number of days? Leap years! We added a certain number of days, not years. (In fact, adding years is not directly supported in timedelta objects.)