issue_comments: 338247940

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https://github.com/pydata/xarray/issues/1621#issuecomment-338247940 https://api.github.com/repos/pydata/xarray/issues/1621 338247940 MDEyOklzc3VlQ29tbWVudDMzODI0Nzk0MA== 1217238 2017-10-20T15:57:14Z 2017-10-20T15:57:14Z MEMBER

I understand the potential issue here, but I think Xarray should follow CF conventions for time, and only treat variables as time coordinates if they have valid CF time units (<time unit> since <date>).

Rich, I know you've been involved in CF conventions and standard names, but I don't think the CF conventions on time apply directly here. These are "time difference" units, which are a distinct type of quantity. And assuredly the period of a wave is a type of "time difference" as well -- it's just not one that is useful to decode into an array with dtype np.timedelta64. Really this is a limitation of the non-ideal support for units in the NumPy ecosystem.

Is there some other sort of metadata we could use to make this distinction of "physical" vs. "human" time differences? Now is a good time to make changes, since we are on the verge of making a major release (v0.10).

Throwing out some ideas, none of which I particularly like (but to be clear, I don't like the status quo either): 1. We could unilaterally stop automatic decoding into timedelta64. (We would need to add a separate helper function that could be called to do these conversions afterwards.) 2. We could add look-up tables that recognize standard_name attributes, either a white-list or black-list for timedelta64 compatible variables. (This would be a first for xarray, and is not something I'm particularly looking forward to maintaining.) 3. We could decode coordinates and data variables differently, only converting coordinates into timedelta64. (This is not entirely ideal either, since it's easy to switch between the two in xarray's data model.)

 
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