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

Jonathan Gregory j.m.gregory at reading.ac.uk
Fri Dec 14 10:35:14 MST 2012

Dear Cecilia, Steve et al.

Steve is right that mostly we use the Gregorian calendar. That is what I meant
mostly when I said that the default is the calendar we use. The real world
is mixed Julian-Gregorian, and I don't think dealing with this calendar is an
issue only for Renaissance historians. I can't give you examples, but I
think it is conceivable or likely that at some point people would want to
record real-world data in CF earlier than the Renaissance, or have already
done so. For instance, what about astronomical data, such as the dates of
eclipses. These are real-world events, on precise dates which are translated
into the mixed Julian-Gregorian calendar. It would not be sensible to insist
on translating real-world dates into the non-real-world proleptic Gregorian
calendar. Hence we have to continue to support the mixed calendar. Abolishing
it in CF is not one of Cecilia's four options, which only concern what the
default should be.

> With respect to the default calendar:

> 3 replace the Julian-Gregorian calendar as default with the
> proleptic Gregorian calendar

I don't think this is acceptable since it changes the meaning of existing data.

> 1 keep the Julian-Gregorian calendar as default (no change)

Since the current default is a pitfall, changing the default would be

> 2 remove the Julian-Gregorian calendar as default, and have no
> default calendar (grid analogy)
> 4 replace the Julian-Gregorian calendar as default with a strict
> Gregorian calendar

I think either of these would work. 2 causes more aggravation. It means that
data which doesn't state the calendar attribute is illegal and will produce
errors, even if it's entirely unproblematic such as "days since 2012-1-1". It
would make CF more intolerant of existing practices than it usually has been.
At the moment, CF accepts COARDS time coordinates; with this change, COARDS
data would not be acceptable.

Hence I would still prefer 4. The aim of this would be to make the default
illegal in cases where there is a serious chance of unsafe time units, and the
obvious criterion seemed to prevent dates before the invention of the Gregorian
calendar. In particular that will exclude reference years of 0 and 1, which
are often problematic. However, I don't feel strongly about it.

As has been said in other postings, CF has its own (non-COARDS) ways of
expressing climatological time.

I think that calendar-based units of time cannot be introduced without a new
syntax for time units, and some rules about how to interpret the cases when
adding months to a reference date gives an impossible date. We could make such
changes to the convention.



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