vschmidt wrote:
> I'm hoping you can help me confirm/deny a bug in pylab's date2num() function.
>
> My assumption (this may be wrong) is that this function is meant to be
> compatible with the MATLAB function date2num(). However, in ipython I can
> execute:
>
> ---------
> import datetime
> import pylab as p
>
> dts = datetime.datetime.now()
> serialts = p.date2num(dts)
>
> print dts
> 2008-11-16 12:03:20.914480
>
> print serialts
> 733362.502325
> ------------
>
> If I then copy this serialts value into MATLAB I get:
>
> ----------
> datestr(733362.502325)
> 16-Nov-2007 12:03:20
> ----------
>
> Note that the year is off by one.
Evidently date2num was designed to be similar, but not identical, to
Matlab's datenum. (The difference might have been inadvertent.) Matlab's
documentation says,
A serial date number represents the whole and fractional number of days
from a specific date and time, where datenum('Jan-1-0000 00:00:00')
returns the number 1. (The year 0000 is merely a reference point and is
not intended to be interpreted as a real year in time.)
And mpl's says,
return value is a floating point number (or sequence of floats)
which gives number of days (fraction part represents hours,
minutes, seconds) since 0001-01-01 00:00:00 UTC
So they simply have a different origin. I find calendars endlessly
confusing, and I make no attempt to delve into them; but I dimly recall
that there is a year 1, but there is no year 0, so perhaps that is an
advantage of the mpl version--not that it should matter in practice.
I think the conclusion is that this sort of date number should be
considered suitable for internal use only, and not used as an
interchange format; for going from one software system to another, one
must use a genuine standard supported by both.
Eric
>
>
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