[Python-Dev] Epoch and Platform

[Guido van Rossum]

Can you explain why you are so anxious to get this resolved (apart
from the beer :-) ?

On Tue, Jun 17, 2008 at 9:26 AM, Curt Hagenlocher <curt at hagenlocher.org> wrote:
> Any chance of an Official Pronouncement on this topic?  It would help
> us greatly -- even if only to figure out who'll be paying for the next
> round of beer.
>
> On Mon, Jun 16, 2008 at 4:38 PM, Guido van Rossum <guido at python.org> wrote:
>> ISTR that we force the epoch to be 1970 on all major platforms -- or
>> perhaps it happens to be 1970 even on Windows when using MS's C
>> runtime.
>>
>> On Mon, Jun 16, 2008 at 4:08 PM, Curt Hagenlocher <curt at hagenlocher.org> wrote:
>>> The documentation for the time module says that "the epoch is the point
>>> where the time starts. On January 1st of that year, at 0 hours, the ``time
>>> since the epoch'' is zero. For Unix, the epoch is 1970. To find out what the
>>> epoch is, look at gmtime(0)."  This confirms that the epoch is
>>> platform-specific.  As such, the only legal uses of the timestamp should be
>>>
>>> 1) comparing with another timestamp to determine elapsed time in seconds
>>> 2) passing to another standard Python library function which expects a
>>> timestamp
>>> 3) as a source of randomness.
>>>
>>> However, the following files in the standard library have hardcoded the
>>> assumption that the Python epoch will always be the same as the Unix epoch:
>>> In gzip.py, method GzipFile._write_gzip_header
>>> In tarfile.py, method _Stream._init_write_gz
>>> In uuid.py, function uuid1
>>>
>>> Additionally, the following files in the standard library have hardcoded the
>>> assumption that the Python epoch will cause timestamps to fall within the
>>> range of a 32-bit unsigned integer value:
>>> In imputil.py, function _compile
>>> In py_compile.py, function compile
>>>
>>> So there's some kind of a potential discrepancy here, albeit a minor one.
>>> This discrepancy can be resolved in one of at least three ways:
>>>
>>> 1) The documentation for the time module is wrong, and the epoch for Python
>>> (at least versions 2.x) should be the Unix epoch.
>>> 2) These library functions are slightly wrong and should be modified by
>>> subtracing an "epoch offset" before doing other calculations. This offset
>>> can be calculated as "time.mktime((1970, 1, 1, 0, 0, 0, 3, 1, 0)) -
>>> time.timezone".
>>> 3) These library files should be considered part of the platform-specific
>>> implementation, and an alternate platform should provide its own version of
>>> these files if necessary.
>>>
>>> Any thoughts on this?
>>>
>>> From the perspective of implementing IronPython, I'd prefer that the answer
>>> is 1 or 2 -- but mainly I just want to be as compatible with "the spec" as
>>> possible, while respecting CLR-specific norms for functionality which is
>>> left up to individual implementations.
>>>
>>> --
>>> Curt Hagenlocher
>>> curt at hagenlocher.org
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>>>
>>
>>
>>
>> --
>> --Guido van Rossum (home page: http://www.python.org/~guido/)
>>
>



-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)