[Numpy-discussion] The date/time dtype and the casting issue

[Pierre GM]

On Wednesday 30 July 2008 06:35:32 Francesc Alted wrote:
> A Wednesday 30 July 2008, Ivan Vilata i Balaguer escrigué:
> > Pierre GM (el 2008-07-29 a les 15:47:52 -0400) va dir::
> > > On Tuesday 29 July 2008 15:14:13 Ivan Vilata i Balaguer wrote:
> > > > Pierre GM (el 2008-07-29 a les 12:38:19 -0400) va dir::

[Pierre]
> > > Well, what about a .tounit(new_unit, reference=None) ?
> > > By default, the reference would be None and default to the POSIX
> > > epoch. We could also go for .totunit (for to time unit)

[Ivan]
> > Yes, that'd be the signature for a method.  The ``reference``
> > argument shouldn't be allowed for ``datetime64`` values (absolute
> > times, no ambiguities) but it should be mandatory for ``timedelta64``
> > ones. Sorry, but I can't see the use of having a default reference,
> > unless one wanted to work with Epoch-based deltas, which looks like
> > an extremely particular case.  Could you please show me a use case
> > for having a reference defaulting to the POSIX epoch?

[Francesc]
> Yeah, I agree with Ivan in that a default reference time makes little
> sense for general relative times.  IMO, and provided that we will be
> allowing an implicit casting for most of time units for relative vs
> relative and in absolute vs relative, the use of forced casting will
> not be as frequent, and that a function would be enough.  Having said
> that, I still see the merit of method for some situations, so I'll
> mention that in the third proposal as a possible improvement.

In my mind, .tounit(*args) should be available for both relative (timedeltas) 
and absolute (datetime) times. I agree that for relative times, a default 
reference is meaningless. However, for absolute times, there's only one 
possible reference, the POSIX epoch, right ? 
Now, what format do you consider for this reference ? 
Moreover, could you give some more examples of interaction between datetime 
and timedelta ?