[Python-Dev] Epoch and Platform

[Curt Hagenlocher]

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
>> _______________________________________________
>> Python-Dev mailing list
>> Python-Dev at python.org
>> http://mail.python.org/mailman/listinfo/python-dev
>> Unsubscribe:
>> http://mail.python.org/mailman/options/python-dev/guido%40python.org
>>
>>
>
>
>
> --
> --Guido van Rossum (home page: http://www.python.org/~guido/)
>