Stephane Raynaud wrote:
Hi,
date2num and num2date perform conversion between datetime and 'days
since 0001-01-01' and vice versa.
For such task, they strictly use ordinal dates for their numeric days,
1 meaning '0001-01-01' by definition.
Thus, date2num(datetime.datetime(1,1,1,0,0,0)) return 1. which is
supposed to mean '1 days since 0001', which is wrong (because it
points to datetime.datetime(1,1,2,0,0,0)).
Since year zero cannot be used (here for time units) because it
doesn't exist, don't you think that the ordinal date
(datetime.datetime(1,1,2,0,0,0).tordinal()) should not be strictly
used as a reference numeric time, but its value-1?
Stephane, yes, what you say makes sense. Long ago I settled on a convention of using "decimal days" referenced to the start of a "yearbase" for time calculations and for plotting variables against time. So if the yearbase is 2008, then noon on January 1 of 2008 is 0.5. The more common convention in oceanography, though, was to label days of the year with a 1-based count and then add the fraction of the day, so what is 0.5 to me is 1.5 to many others. In this case, where the time scale origin (the start of the yearbase) might be in the middle of one's time series, the decimal day definition is clearly superior (at least to me). But in the case of the matplotlib dates module the distinction is less important, because the origin is quite arbitrary and will almost always be far smaller than the minimum of the range plotted. The datenum is mainly useful for calculations, not for direct display. Note that the datetime module (and therefore mpl.dates) simply doesn't work for BC dates.
Personally, I would be perfectly happy to implement your suggestion so that the reality would correspond to the dates module docstring; but maybe this would break some user code, so others might prefer to modify the docstring to reflect the present behavior instead.
John, I suspect you wrote the dates module and use it heavily--what do you think about the two methods of bringing the docstring and the behavior into alignment? Any problem with fixing the behavior?
Eric