Hi Chris, and all:
On 1/11/2013 2:37 PM, Chris Barker - NOAA Federal wrote:
On Fri, Jan 11, 2013 at 9:00 AM, Aleksandar Jelenak - NOAA Affiliate
<[email protected]> wrote:
Here's the modified proposal for the datetime_iso8601 standard name:
...
String representing date-time information according to the ISO
8601:2004(E) standard.
I think we should NOT adopt a string option for datetime variables.
To quote Jonathan Gregory:
"""
In CF we have always applied the
principle that we only add to CF when there is a need to do so, i.e. there is
a use-case for something which cannot already be represented in CF
"""
We already have a way to encode datetimes in CF-netcdf.
Yes, but <time since date> is not as good as <date> as an encoding. The
<time since date> is a result of cramming calendar handling into a units
package.
I would advocate both should be allowed.
I believe this proposal resulted from the discussion about adding a
more flexible approach to datetimes in the CF Data Model. I think
that's a good idea, but separate from what encoding is used in
CF-netcdf. ( see my recent note for more detail about the difference
between and encoding and a data model ).
1) Having multiple ways to encode the same data in file format adds
complication to all client code -- client code would need a way to
process both ISO strings and "time_unit since datetime"
client code already has to parse the "date" in "time since date". So
theres no extra code involved.
2) Any client code that can process ISO strings is likely to need to
convert them to a client-specific datetime representation anyway, in
order to plot, calculate with, etc them.
3) Any client library that can process ISO strings is very likely to
be able to also work with "time_unit since datetime" encoded data
anyway -- and it had better, as that encoding is part of the standard
anyway.
As a result, we would be complicating client code, and getting no new
functionality.
We get new functionality in that "date" is clearer than "time since
date", and more likely to be correctly understood by non CF specific
software and users of our data in 100 years when theres no more CF
discussion group to help people out.
when you have non-standard calendars, the difficulty is compounded many
times over. How many seconds since 1970 is April 3, 2045 at 1:13 am in
the no-leap calendar? Are you sure? What if you could just put into your
file "2045-04-03T01:13:00" ?? Even rocket scientists can do that ;^)
The one advantage I can see at the moment is that simple, non-CF-aware
clients, like ncdump, could easily present a nice human-readable
format. But I don't think that is worth the additional complication.
Ideally file encodings should be as independent as possible from
libraries and applications. We have historically had an unfortunate
dependence on the udunits reference library for date parsing. We are
slowly unwinding that dependence. I think in this case widening the
allowed encoding for datetimes is well worth the complication.
Regards,
John
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