Hi Richard,
I have some opinions and discussion on some of your points.
On 7/1/2013 8:41 AM, Steve Hankin wrote:
Yo may want to follow/participate on this discussion
-------- Original Message --------
Subject: [CF-metadata] Non-real-world calendars
Date: Mon, 1 Jul 2013 13:26:22 +0000
From: Hattersley, Richard <[email protected]>
To: [email protected] <[email protected]>
Hi everyone,
I'd like to propose a trac ticket or two to clarify the meaning when
using alternative calendars. But before I do that I'd like to check
for community opinion (or even consensus!?) ...
For one thing, there should be a definition of "real world calendars",
shouldn't there.
1. Time zones should be excluded/banned when using non-real-world
calendars. For example, the statement in section 4.4 of "if the time
zone is omitted the default is UTC" should not apply.
2. The "months since" and "years since" semantics for non-real-world
calendars need defining/outlawing. e.g. The UDUNITS definition of a
year as 365.242198781 days makes no sense at all for a 360-day
calendar, but in this particular case a year could be unambiguously
defined as 360 days.
For each calendar, there's a year length in days. The definition of
"years-since", etc. would always flow from the calendar definition
that's in place. So if the definition of time is noleap (equivalent to
Udunits common_year), then years_since or days-since would be computed
using a 365-day year. Is that what the Udunits library does? "Month"
is inherently ambiguous as discussed in the CF document, but would be
1/12 of a year. In CF, the definition of a year as 365.242198781 days
doesn't apply to any of the calendars, because it doesn't relate to
calendar months/days/hours etc. (Does the Udunits library use that
number? How?)
All of the calendars in CF section 4.4.1 have definitions that allow
software to convert between time coordinates and date-strings using the
unit, time origin and calendar. They're consistent within themselves.
Each one implies a number of days/fractional days per year.
Ansley
3. The year-zero semantics for non-real-world calendars need defining.
From section 7.4, "Year 0 may be a valid year in non-real-world
calendars".
I have some further questions concerning real-world calendars, but as
with all things dealing with the real world they are a little more
messy so I'll save them for another post.
Richard Hattersley
Benevolent Dictator of Iris - a CF library for Python:
www.scitools.org.uk/iris <http://www.scitools.org.uk/iris>
*Met Office* FitzRoy Road Exeter Devon EX1 3PB United Kingdom
Tel: +44 (0)1392 885702
Email: [email protected]
<mailto:[email protected]> Web:
www.metoffice.gov.uk <http://www.metoffice.gov.uk/>
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