[CF-metadata] CDM calendar date handling

From: Jonathan Gregory <j.m.gregory>
Date: Wed, 24 Aug 2011 16:23:39 +0100

Dear John

> since im going to propose some grammar that we will be stuck with
> for the next 50m years ...
:-)

We've talking about the syntax which describes the reference date now, is
that right? That is, the complete syntax is
N [calendar] UNITS since DATE
and we have so far said that it's not a good a idea for N to be -50 million
+- small increments, so instead we want to put the large offset into DATE.
Usually DATE is a string as described in your section "W3C profile of ISO8601".

You suggest
> "01-01-01 12:00 epoch 50m BCE"
> "01-01-01 12:00 reference 50m BCE"
but for this purpose I think we could assume we mean 0Z on 1 Jan. The aim is
to have an offset of a large number of years. Offsets within the year can be
dealt with as usual by the N [calendar] UNITS. So I suggest that DATE should
be allowed to be "N BCE" or "Nk BCE" or "NM BCE" (big M rather than little m
for mega) where N is an integer interpreted as calendar years e.g.
"n calendar years since 65500k BCE" is for counting time in months since the
end of the Cretaceous. :-) To be more helpful you could accept floating-point
numbers if k or M was specified, so we could have "65.5M BCE" in this case.

> >The other kind of example of palaeoclimate is the
> >one where large periods of time are spanned by the time axis

So now we are taking about the "N calendar UNITS" part, when that part means
a large number of years. I think this format:
> "50 calendar Myears since 1980-01-01"
is the best one. The word "calendar" is useful to make clear it is not udunits
years. Also, it's good to have a reference explicitly. For example, "before
present" sometimes means before 1950, and it's nice to have that spelled out.

> >it would be convenient to have time-bounds for a cell of e.g. "1 calendar month
> >since reference" and "2 calendar months since reference", with the time-coord
> >of "1.5 calendar months since reference".
>
> it seems like a more informative coordinate value would be, eg "Feb
> 2001" when the bounds are [1,2] calendar months since 2001-01-01 ?

That is true, that is what it means in this case. (The old GDT convention had
a syntax for specifying a subset of the components of time in this way.) But
it feels to me like we'd be doing something rather more complicated now if we
start to allow months to be named absolutely. Also, even though it is
arbitrary what you choose, sometimes it is used as an exact number, for
example if you differentiate in time. You could translate 1.5 months since
2001-01-01 into an exact datetime. It means half-way between 2001-02-01 and
2001-03-01.

Best wishes

Jonathan
Received on Wed Aug 24 2011 - 09:23:39 BST