[CF-metadata] CF calendars (was: problem with times in PSD dataset)

From: John Caron <caron>
Date: Mon, 17 Dec 2012 18:00:18 -0700

Hi Chris:

On 12/17/2012 4:50 PM, Chris Barker - NOAA Federal wrote:
>> I think a solution shouldn't break current files which followed what had
>> been a standard for a long time (however ill-advised the standard was). I
>> don't have a good sense of what might break if the standard changed in terms
>> of software so I can' speak for all users but I do know many people have
>> downloaded our mean files with 1-1-1 base dates
>
> Hmm -- this seems to contradict Steve's "no known users".

there are no known users using mixed gregorian/julian who actually need
to use it (cross the transition date line). if there are any such users,
please speak up, so we can think about your use case.

>
> BUt anyway, I wonder if folks currently using such files are actually
> getting the "correct" results, when they do.

those using udunits get the correct result even when they cross the
line. i suspect its correct because using a different implementation
(joda time library) gets the same results for the small sample i have
tested.

>
> For instance, in my naivet?, I never thought about calendars when
> using netcdf files -- I simple converted "something since some date"
> to Python datetime objects using python's datetime package, which uses
> "An idealized naive date, assuming the current Gregorian calendar
> always was, and always will be, in effect"
>
> So I would have gotten the "wrong" results.

it sounds like you were using what is called proplectic gregorian. so
you would have gotten wrong results on the PSD dataset, which uses "time
since 1-1-1". Thats the argument for not using mixed gregorian, that
simple things arent simple.

>
> I've lost track whether UDunits does the mixed calendar right, but I'm
> sure not all libraries one might use do.
>
> So is this a use case se need to preserve?

we have datasets out there who are using "time since 1-1-1"
unfortunately. Theres no doubt we need to leave those "working"; and
even that they can continue their dangerous ways if they want to
explicitly include a calendar attribute in CF-1.7 convention file. The
proposal is just to change the default in CF-1.7 to use a more common
calendar, and to encourage people not to cross the time warp zone
unecessarily.

John
Received on Mon Dec 17 2012 - 18:00:18 GMT