The datetime module does not support time zones whose offset from UTC is
not an integer number of minutes.

The Olson time zone database (used by most UNIX systems and Mac OS X)
has a number of time zones with historic offsets that use second
resolution (from before those locations switched to a rounded offset
from GMT).

If you are working purely with the Python date time module, you can
round these offsets to whole numbers of minutes and get consistent
results (this is what the pytz module does), there are problems if you
want to interact with other programs using the Olson database.

As an example, PostgreSQL can return dates with sub-minute UTC offsets
and it would be nice to be able to represent them exactly.

The documentation doesn't explain why offsets need to be a whole number
of minutes, and the interface (returning timedeltas) doesn't look like
it'd need changing to support second offsets.  I wouldn't expect any
more complexity in the code to support them either.

As far as I can tell, the TZ offset code can be simplified by eliminating conversion from timedelta to int and back in utcoffset() and dst() methods of time and datetime objects.

The only reason for the restriction that I can think of is that some text representation of datetime only provide 4 digits for timezone. The behavior of formatting functions, can be preserved while allowing fractional minute offsets elsewhere.

On the other hand, given that this issue did not see a single response in more than a year, it seems that there is little interest in fixing it.

This proved to require a lot of changes to C implementation because most of timezone arithmetics is done using integer operations with offset in minutes. It is easy, however to do this in pure python implementation which can be found at http://svn.python.org/view/*checkout*/sandbox/branches/py3k-datetime/datetime.py .

Marking this "easy" to attract volunteers who may want to do Python prototype.

I am attaching a patch against sandbox version of datetime.py.  With this patch, there is a single place where subminute offset is rejected, _check_utc_offset() function.  I have also added "whole minute" asserts in places where sub-minute part of the offset is discarded or assumed 0.

I am attaching a rough patch which removes timedelta -> int minutes -> timedelta round trips from utcoffset handling code.  I think the result is an improvement, but needs more polishing.

Mark,

Do you think this is worth pursuing?  I am not intending to add support for sub-minute offsets yet, just pass offsets around as timedeltas internally without unnecessary conversions to int and back.

I am attaching a test coverage file for the patched datetimemodule.c.

It looks like I attached coverage for the original file.  See new-datetimemodule.c.gcov for coverage after the change.

Uploaded issue5288.diff to Rietveld:

  http://codereview.appspot.com/1698050

issue5288a.diff addressed comments from Rietveld.

C code changes eliminating round-trips between timdelta and int offsets committed in r82642 and Python code changes committed to sandbox in r82641.

Now the requested behavior change is easy and I am about +0.5 on relaxing the offset checks to allow seconds in the offsets and about +0.1 on removing the checks altogether and allowing arbitrary timedeltas even over 1 day.

The only substantive question in my mind is how to handle printing in formats like %z which only provide 4 digits for UTC offset.  Two options seem reasonable: a) truncate sub-minute offsets; and b) follow RFC 3339 recommendation and translate the time into the nearest whole-minute timezone.

I am slightly is favor of (a), but this whole problem may be a reason to reject this RFE.

Digging up an old issue but I am also interested in seeing this enhancement. Specifically to represent GPS time which is (currently) 16 seconds ahead of UTC.

Is it practical to implement GPS time as datetime given that we don't have support for leap seconds?

GPS time doesn't include leap seconds so I think datetime is a good representation. If datetime doesn't know about leap seconds then there would still be some issues with finding the timedelta between a GPS time and a UTC time straddling a leap second but I guess a similar issue also exists with two UTC times.

For my application all the times I am dealing with are in a short period and will have the same UTC offset so its a little easier, I can probably avoid most of these issues.

However, wouldn't it be possible to implement the general case with a non-constant utcoffset function (and new fromutc implementation) in the tzinfo class? Of course there is no way to properly handle UTC times more than 6 months or so in the future...