[CF-metadata] CDM calendar date handling

From: Jonathan Gregory <j.m.gregory>
Date: Wed, 24 Aug 2011 13:23:20 +0100

Dear John

> It seems to me it would be better to somehow denote the "epoch"
> seperately, because its kind of silly keeping track of # millisecs
> between two dates separated by 50 million years. plus its hard.
>
> what about:
>
> "01-01-01 12:00 epoch 50m BCE"
>
> where the "epoch 50m BCE" is probably just carried along in the
> string representation of the date.

I agree that that's a sensible way to do it for dates which are separated
by relatively short periods in an epoch which is a long time ago. However
it could be useful if the epoch wasn't just a string. If it was a longinteger
number of calendar years (relative to the normal year 1) you would be able
to interconvert dates expressed relative to different epochs. That is, you
could tell that 10 calendar Myears since 01-01-01 epoch 20M BCE means the
same as 0 calendar years since 01-01-01 epoch 10M BCE. This conversion would
be accurate provided the time unit in calendar years is stored as an integer.
Accuracy is lost if it is turned into floating-point milliseconds, I suppose.

The other kind of example of palaeoclimate is the
one where large periods of time are spanned by the time axis e.g. if you
had a timeseries whose time axis spanned all the geological periods since
600 Myear ago. (That would probably not be from a GCM! - but it could be from
geological data or other Earth system models.) That sort of axis would like
to use units of calendar Myear and they need to be stored accurately.

About the multiples of calendar units: You have said they should be integers.
However, perhaps fractions could be rather useful. For example, we have the
recurrent question of where the time coordinate should be positioned in the
interval if the data actually applies to the interval rather than the instant.
It's a bit arbitrary and the usual advice is to put it in the middle. For this
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".

Best wishes

Jonathan
Received on Wed Aug 24 2011 - 06:23:20 BST