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

From: Jonathan Gregory <j.m.gregory>
Date: Fri, 7 Dec 2012 13:53:10 +0000

Dear all

I think that we should be very cautious about backwards-incompatibility, even
when changing CF versions. I would be worried about making a change which
means metadata would have a different meaning according to which version of
the convention is used. I know in principle it is safe because the file states
the version, but I don't think we can depend on the version to be coded
correctly, or on software to make correct use of it. Although these are errors,
we ought nonetheless to design the convention to be as robust as possible in
an error-prone world.

Therefore I don't think it is safe to change the the default calendar from
gregorian to proleptic_gregorian, because that could change the date you
get for a given time coordinate. However, I do think it would be safe to make
it illegal to use the default calendar for reference dates earlier than
1582-10-15, or for negative time coordinates (to make sure that dates before
1582-10-15 cannot be specified). In that case, real-world data for which there
is no problem would be unaffected, but software would reject time coordinates
whose interpretation is problematic. Software could provide an option whereby
the user could state what the calendar actually is, when not specified, in
order to override the error if the user knows what it truly is. Alternatively,
they could easily repair the file with ncatted or some such tool.

It should still be possible to use the real-world calendar before 1582-10-15,
because CF might be used for real-world historical data earlier than that date,
but it would be necessary to state the calendar explicitly.

Best wishes

Jonathan
Received on Fri Dec 07 2012 - 06:53:10 GMT