[Numpy-discussion] NumPy date/time types and the resolution concept

[Pierre GM]

On Monday 14 July 2008 12:50:21 Francesc Alted wrote:
> > A very useful point that Matt Knox had coded is the possibility to
> > specify starting points for switching from one resolution to another.
> > For example, you can have a series with a 'ANN_MAR' frequency, that
> > corresponds to 1 point a year, the year starting in April. When
> > switching back to a monthly resolution, the points from January to
> > March of the first year will be masked.
>
> Ok.  Ann was also suggesting that the origin of time would be
> configurable, but then, you are talking about *masking* values.  Mmm, I
> don't think we should try to incorporate masking capabilities in the
> NumPy date/time types.

Francesc,
In scikits.timeseries, we have 2 different objects:
* DateArray, which is basically a ndarray of integers with a given 'frequency' 
attribute.
* TimeSeries, which is basically the combination of a MaskedArray (the data 
part) and a DateArray (which keeps track of the date corresponding to each 
data point. TimeSeries object have the resolution/origin of the companion 
DateArray, and when they're converted from one resolution to another, some 
masking may occur.

My understanding is that you intend to define an object similar to DateArray. 
You want to define a new dtype (datetime64 or other), we used yet another 
class instead, Date. A dtype would be easier to manipulate, but as neither 
Matt nor I were particularly experienced with that at the time, we followed 
the simpler approach of a class...

> [N]timeunit
>
> where ``timeunit`` can take the values in:
>
> ['y', 'm', 'd', 'h', 'm', 's', 'ms', 'us', 'ns', 'fs']
>
> so, for example, '14d' means a resolution of 14 days, or '10ms' means a
> resolution of 1 hundreth of second.  Sounds good to me.  What other
> people think?

Sounds pretty cool and intuitive to use. However, writing the conversion rules 
from one to another will be a lot of fun. Take weekly, for example: that's a 
period of 7 days, but when does it start ? On a monday ? Then, 12/31/2007 was 
the start of the first week of 2008... OK, we can leave that problem for the 
moment...