[Numpy-discussion] dealing with datetime UTC vs linear time

Mark Wiebe mwwiebe@gmail....
Mon Jun 6 17:12:56 CDT 2011


On Mon, Jun 6, 2011 at 3:47 PM, Mark Dickinson <mdickinson@enthought.com>wrote:

> On Mon, Jun 6, 2011 at 9:24 PM, Mark Wiebe <mwwiebe@gmail.com> wrote:
>
> Sorry for being dense, but:
>
> > C. Add a metadata item which chooses between "UTC" and "TAI". For seconds
> > and finer, converting between UTC and TAI units is safe, and for minutes
> and
> > coarser, is unsafe. UTC involves leap-seconds
>
> This bit I understand, but:
>
> > and an ambiguity-resolution in the few cases that need it,
>
> Where does the ambiguity come from?  (Assuming we're not more than 6
> months into the future.)  There would be ambiguity with POSIX time,
> but doesn't UTC manage to specify any point in time unambiguously?
>

If you add one minute to an added leap-second, you end up at second 60 in a
minute which has only 60 seconds, and you have to fall back to second 59 or
forward to second 0 of the next minute. Also, if you add a minute which
should land where a leap-second was removed, you have to fall back to second
58 or second 0 of the next minute.

> [...]
> > Does this design sound reasonable to people?
>
> Python's datetime and timedelta types use POSIX time (i.e., just
> pretend that leap seconds don't exist and hope that everything works
> out okay).  Would this cause problems when converting between Python
> and NumPy types?


Converting from the Python type to the NumPy type has a problem for removed
leap-seconds (of which there are currently none), and converting from the
NumPy type to the Python type has a problem for added leap-seconds. To allow
for round-tripping to/from python objects, I think I would just make it
return the underlying 64-bit number instead of a Python datetime object.
This is also what it does for years outside of the range Python accepts.


> I guess part of the point of this work is that with
> full datetime support in NumPy the need for such conversions would be
> relatively rare, so maybe there's no real problem here.
>

Yeah, as long as the calendar-related API is sufficient, I think that's
true.

But having thought quite hard about some of these issues for Python's
> datetime module recently, I'm sceptical that leap second support is
> really that valuable, and fear that having it turned on by default
> will cause nasty surprises for those who have no need to think about
> leap seconds and write code that assumes that a minute has 60 seconds.
>

I think that by having some default ambiguity-resolution rules for leap
seconds, things will generally work out as people expect. On the other hand,
with POSIX time, anybody that wants things precise with respect to
leap-seconds is automatically excluded. The potential surprises from
leap-seconds seem milder than the surprises from time zones and the
multitude of possible calendars out there. I found it disconcerting that
datetime.datetime.now() returns local time with no time zone indication on
it. Here's how I've resolved that issue in NumPy:

>>> datetime.datetime.now()
datetime.datetime(2011, 6, 6, 17, 10, 32, 444658)
>>> np.datetime64('now')
numpy.datetime64('2011-06-06T17:10:41.000000-0500','us')

Cheers,
Mark
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