[CF-metadata] New standard name: datetime_iso8601

From: John Caron <caron>
Date: Wed, 20 Mar 2013 08:58:20 -0600

Hi all:

I guess I started this messy discussion, then invited everyone to drink
too much and say whatever they wanted. The conversation gets a bit loud
and fuzzy. So now we've switched back to caffeine and the sober
realities of deciding on grammars in the context of a specific proposal
that seems to be acceptable to all.

Since there were a lot of "whats the problem" comments, I will just give
my motivation for this topic.

There are two things you want to do with datetimes: 1) specify an
instance on the datetime coordinate axis. 2) compute the time difference
between two instances (a calendar is needed to compute time differences).

Our current "period since date" representation is better for the second
case. A direct representation of the time instant (like ISO8601) is
better for the first case. I suggest we allow both, as a modest improvement.

Mostly my POV is a library implementer, where the following issues are
in play:

   1. It turned out to be a mistake to rely on udunit library for
datetime processing. I think we no longer have an official reference
library for this (?).

   2. Non-standard calendars are non-trivial.

   3. Theres little or no extra work to implement, because we already
have to parse the date string in "period since date" representation.

   4. The time instant representation doesnt solve the non-standard
calendar, since you will still want to be able to compute time differences.

Really the main advantage is that data writers are less likely to make a
mistake specifying

1952-08-15T00:00:00Z

than

2434567 days since -4713-01-01T00:00:00Z.

and you shift the burden of calculating time differences onto the software.

Note that Im just advocating for adding this as an option, not replacing
the current standard.

John

PS: Jonathan asks: "how can we reduce the possibility of these
mistakes"? We need a reference library for datetime that reads CF files
and calculates both instances and time differences, and spits them out
in user-configurable ways so that data writers can double check that
their files are written correctly. It has to handle all the non-standard
calendars that are in CF.
Received on Wed Mar 20 2013 - 08:58:20 GMT