@peterkuma thanks for the detail, I was looking at ISO 8601-2014 (accessed in 
2018) ... and the treatment of years -9999 to 0 has clearly be considerably 
enhanced in the 2019 version, especially in the extension (8601-2). I've just 
downloaded a new version. 

The NetCDF Java library has quite extensive calendar support .. but the only 
thing I could find in the NUG that our convention references was a link to the 
CDL functionality which I've illustrated above. I agree completely with 
@lesserwhirls that the NetCDF libraries, and libraries in general, should not 
be treated as a standard or convention, but we have to consider the 
consequences of recommending anything that conflicts with a widely used 
library. 

Using the new explicit  ISO 8601 form for dates may help to make the 
distinction between the two mappings clearer, with `1YB` equivalent to `1 BC` 
and `0000-01-01` as  1 year before `0001-01-01`. A complete date and time looks 
like this: `1985Y4M12DT23H20M30S`, which would require some extra parsing code. 
There is, however, a lot of flexibility in the ISO standard, as you might 
expect. I have doubts about an approach which would require users to deal with 
the full range of options.


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