Use np.datetime64[ns] for dates between years 1678 and 2262?

[Manangka]

In GitLab by @Huite on Mar 10, 2019, 15:21

To support years before 1679 and after 2262, dates are converted to cftime objects.
Turns out this isn't ideal, as cftime objects are not nearly as ubiquitous as the np.datetime64[ns] dtype. Currently, neither plotting nor resampling will work out of the box.

The xarray docs mention that for suitable dates, it defaults to np.datetime64[ns], since this integrates nicely with e.g. pandas or matplotlib.
http://xarray.pydata.org/en/stable/time-series.html#creating-datetime64-data

Mentioned there, the following issues address some problems:

• Adding resample functionality to CFTimeIndex pydata/xarray#2191
• WIP: sketch of resample support for CFTimeIndex pydata/xarray#2458
• holoviews / bokeh doesn't like cftime coords pydata/xarray#2164

I suggest we do the same: I estimate that over 90% of use cases are within the mentioned bounds.
As far as I can tell, there are no good & consistent solutions (yet) for date ranges outside of those bounds: pandas seems pretty stuck to nanosecond resolution. cftime functionality will be expanded to support plotting, but will likely remain a "second-rate" citizen for quite some time. (Just to name an example: lack of real pandas support will mean that xarray's .to_dataframe() method probably be complicated.)

Defaulting to np.datetime64[ns] introduces the least complexity in the most cases: if you're dealing with long timespans, you're going to run into trouble somewhere else anyway.

Concretely, my idea is:

• If within bounds, return np.datetime64
• Else, return cftime, since it supports a good bit of xarray functionality (with 0.12 release)
• Add a use_cftime option to idf.load, similar to: Add use_cftime option to open_dataset pydata/xarray#2759, so all DataArrays have consistent datetimes, when you're working with long timespans.

[Manangka]

In GitLab by @visr on Mar 10, 2019, 19:24

closed via merge request !20

[Manangka]

In GitLab by @visr on Mar 10, 2019, 19:24

mentioned in commit 0f723eb