[CF-metadata] 'months since' and 'years since' time units

From: Chris Barker - NOAA Federal <chris.barker>
Date: Fri, 19 Oct 2018 12:53:13 -0700

I remember this coming up on this list a while back.

Yes, the concept of e.g. monthly averaged data is useful.

But in that case, a ?month? is really a category, not a continuous time
variable.

So we need a different way if describing it than a time access with units
of month...

-CHB

Sent from my iPhone

On Oct 19, 2018, at 8:45 AM, Jim Biard <jbiard at cicsnc.org> wrote:

I like how you have described the issue, Chris.

Using month in anything except a 360-day calendar (assuming the month is
defined correctly for that calendar) produces erroneous results if you try
to do anything but math that remains in those units - such as convert a
month count to a date. The same goes for physical years.

And yet... If we are writing data collected on monthly, seasonal, or other
large-scale time binnings (when doing climatologies, for example), month
and year become regularized - that is, they become metric within the
context of that data, and it would be so much more human-friendly to be
able to work directly in months, seasons, and years, regardless of whether
that is a real year, a 365 day year, or a 360 day year.

It seems to me that time recorded in "month and larger" terms really
represents a different thing, separate from 'time' as defined by CF. I see
two different solutions that could be based around a new standard name of
'date', as Martin Juckes suggested.

   - Declare that the values for a standard name of 'date' are date strings
   following a YYYY-MM-DD format (and perhaps something for megayears, etc
   that would be useful for paleo people).
   - Declare that the values for a standard name of 'date' would use units
   of calendar_month, calendar_season, and calendar_year (or whatever names
   people like best). These units are entirely convertible between each other,
   and would need to be added to UDUNITS. UDUNITS would not convert between
   these units and time units. People would be free to write code that could
   do the conversion between time and date, but there wouldn't be any
   particular way that would be considered standard.

Grace and peace,

Jim

On 10/19/18 11:13 AM, Chris Barker - NOAA Federal wrote:

Calendars are a mess? both because the earth?s rotation is not
human-frendly, and because of human legacy.

So we need to accept that, and not try to use calendars as though they
are logical units for time.

I like to think about it this way: there are time operations: ?this
much time has passed?, and there are calendar operations: ?this day
next month?.

Calendar operations require a lot of machinations and a well defined
calendar. Time operations are straightforward and behave ?sensibly?
with the usual math.

Also ? operations belong in libraries, not data files. CF should use
only well defined units.

Personally, I think udunits should never have defined ?months? or
?years? as a time unit. But in any case, CF can at least highly
discourage, if not ban, their use.

Possible exception: for ?artificial? Calendars, a month can be well
defined, but then the unit should be called something like
?30_day_month?, other well defined name.

I know that CF refers to udunits for unit definitions, but we really
need to either allow exceptions or periodically update udunits to meet
the needs of CF.

-CHB



On Oct 19, 2018, at 5:13 AM, B?rring Lars <Lars.Barring at smhi.se>
<Lars.Barring at smhi.se> wrote:

Dear all,

I agree with Jonathan's wish for a more well-behaved Earth in the
planetary system :-)

However, awaiting this I think that we have two issues before us:

1. The fact that different datasets fundamentally are based on
different length of a year, while Udunits defines a year to be the
real world tropical year.

2. The common language notion of months of different length (and for
February varying), while Udunits defines the length of a month to be
one twelfth of the real world tropical year.


I fully agree that for datasets from one source (having one calendar)
the warning against using "month_since..." and "year_since..." is
very well motivated. But, as Martin points out, when combining data
based on different calendars the easiest /best/most relevant way is to
use one of these. Having said that I will in turn go into some detail
with the two issues.


1. Regarding the length of the year, which is what my previous posts
concerned, I think that we have three options:
A) Convince Udunits to change the behaviour of the year unit to
depend on the calendar. To my mind this would actually solve the
problem stated in the initial post of this thread. Whether this option
s possible, have a chance to fly, or is something that the CF
community actually want I do not now. But here is the idea for you to
consider. If this solution is implemented, it would mean changing the
text in Section 4.4.
B) In the CF Conventions specify that the definition of the length of
a year deviates from the one used in Udunits. This would mean changing
the text in Section 4.4. Again, once this change has penetrated into
software libraries, this would solve the initial poster's problem.
C) Leave things as they are, preferably with some additional language
in Section 4.4 to explain when unit year might be useful and when not
to use it.

Of course, there might be other options that I have not though of.
Personally I advocate A) or B), because to my mind the impact of
having defined different calendars is not fully implemented in CF
because the different model calendar _do_ imply different length of
the year. Moreover, at least for the 360_day calendar the Udunits
definition of a month will then coincide with what is expected from
that calendar.

2. The obvious root of the problem here is of course the different
months' lengths in the real world calendar. Having said that, what
adds to the confusion is that Udunits has chosen the name "month" for
the unit year/12. I will put aside the problem of different month
lengths and only consider the Udunits months, which I here will call
"month12" for clarity, and focus on the implications of having data
from different calendars being merged (think ensemble summary) and
described using "month12_since" (where the time coordinate takes on
integer values). For all calendars, but 360_day, this implies that a
fractional day goes into the month12ly statistic. If this is a problem
one may one may postulate that the fractional day belongs to the
month12 where the larger part belongs. If this is complemented with
the notion of calendar_month it may be reasonable to restrict at
least the latter, or both, to only take on integer values, to avoid
hairy questions like ones that Jonathan asked "What does 1
calendar_month since 31 January mean? What does 7.23 calendar_months
since 31 January mean?"


Kind regards,
Lars

PS. For those of you without a Scandinavian keyboard; you might
relieved to know that my first name is Lars (and B?rring is family
name, despite what the email alias might suggest)


________________________________________
Fr?n: CF-metadata [cf-metadata-bounces at cgd.ucar.edu] f&#246;r Taylor,
Karl E. [taylor13 at llnl.gov]
Skickat: den 19 oktober 2018 06:14
Till: cf-metadata at cgd.ucar.edu
?mne: Re: [CF-metadata] 'months since' and 'years since' time units

Dear all,

I won't make any recommendations for udunits, but I will comment on the
CF-conventions.

In general, I think we should discourage usage of calendar month as a
unit of measurement because for the real world, these are only defined
for 12 special periods each year (the beginning to the end of each
calendar month) and the "
<Kindregards,LarsPS.ForthoseofyouwithoutaScandinaviankeyboard;youmightrelievedtoknowthatmyfirstnameisLars(andB?rringisfamilyname,despitewhattheemailaliasmightsuggest)________________________________________Fr?n:CF-metadata[cf-metadata-bounces at cgd.ucar.edu]f&#246;rTaylor,KarlE.[taylor13 at llnl.gov]Skickat:den19oktober201806:14Till:cf-metadata at cgd.ucar.edu?mne:Re:[CF-metadata]'monthssince'and'yearssince'timeunitsDearall,Iwon'tmakeanyrecommendationsforudunits,butIwillcommentontheCF-conventions.Ingeneral,Ithinkweshoulddiscourageusageofcalendarmonthasaunitofmeasurementbecausefortherealworld,theseareonlydefinedfor12specialperiodseachyear(thebeginningtotheendofeachcalendarmonth)andthe>unit"
is not constant throughout the year.
Nevertheless there are some good arguments for considering adopting a
special calendar month unit by the CF conventions, but only for limited
and very specific purposes. (I'll refer to these new units as
"calendar_months" in the following, with the understanding that the unit
will depend on the calendar adopted and will in general vary for
different months of the year and depend on whether or not the year is a
leap year.) Here are reasons (already articulated by others) why we
might want to adopt "calendar_months" as a unit:

1) there are existing data sets with monthly-mean data and a
time-coordinate that is supposed to indicate how many calendar months
have passed since some base month/year. Such data sets are not easily
accommodated by CF.
2) for some judiciously selected reference times, coordinates expressed
in "calendar_months since" can be easily generated without the help of
calendar-aware time-translation algorithms. For example, given units of
"calendar_months since 2001-01-01" we can trivially generate the
coordinate values for the middle of each month: 0.5, 1.5, 2.5, etc. It
would be much more difficult to generate the coordinate values in units
of "days since ...".
3) monthly mean data sets with time coordinates based on different
calendars can more easily be compared because if all the data sets
adopted the same reference time, then comparable months would have the
same coordinate values, independent of the actual month length defined
by different calendars. [In contrast, if the time coordinate were given
in units of "days since ... ", the coordinate values would depend on the
calendar.]

If we define a unit of "calendar_months since ..." we would need to
address Jonathan's point that it is not immediately obvious how to
handle fractions of months and reference times different from the
beginning of a month. One approach we should consider is to restrict
use of calendar_month units to datasets reporting monthly values only
(not, daily, annual, hourly, etc.) Also for simplicity we could
restrict the reference time to be the beginning of a calendar year
(i.e., January 1 at 0:0:0 for some specified year). If we did this, it
would be relatively easy to define what we mean by fractions of months
and it would also be easy to generate the values needed to define the
time-coordinates and the cell_bounds or the bounds needed to define
climatologies. [It would be almost as easy if we allowed the reference
time to be the beginning of *any* month, but let's consider the more
restrictive "beginning of a year" option first.]

I note that using calendar_month units to report data at intervals other
than monthly intervals offers no advantages. For example different
fractional month increments would have to be used to report data at an
invariant daily interval. This would seem to introduce complexity to a
simple time-coordinate and is why I would restrict use of calendar_month
units to monthly data.

Since months are not all equal in length, the interval of times
represented by the same fraction may differ between months. For a
Gregorian calendar, half-way through the month of January would be noon
on the 16th of January (15.5 days from the beginning of the month),
whereas half-way through the month of June would be the 16th of June at
00:00:00 (15.0 days from the beginning of the month. Thus 0.5 months
since 2001-01-01 would be 2001-01-16 12:00:00 and 5.5 months since
2001-01-01 would be 2001-06-16 00:00:00. Also note that the date
corresponding to the middle of February depends on whether the year is a
leap year or not.

Of course for a different calendar (e.g., 360_day), the dates
corresponding to 0.5 months since 2001-01-01 and 5.5 months since
2001-01-01 would be different from those for the Gregorian calendar.

The bottom line is that under the above restrictions, it is easy to
convert fractions of months to dates (and also to alternative units such
as "days since ...").

Common types of "monthly" data sets are:

1) reports of monthly statistics computed from multiple samples within
each month (e.g., means, standard deviation of daily values, maximum
daily mean; requires cell_bounds)

2) reports of monthly accumulations (e.g. monthly precipitation amount;
requires cell_bounds)

3) monthly climatologies (requiring climatology attribute pointing to
climatology bounds)

For all of the above the bounds coincide with the beginning and end of
each month so with the reference time restricted to the beginning of a
year, the bounds will all be integer values of "months since". The
coordinate value for any of the above can be assigned any value in the
interval defined by the bounds. For monthly statistics one might choose
the middle of each month so coordinate values would be numbers like 0.5,
1.5, 2.5, etc. For monthly accumulations one might prefer to use the
time at the end of each interval to represent the coordinate value
(e.g., 1.0, 2.0, 3.0, etc.).

I understand that defining a unit of calendar_months is not compatible
with udunits, but I think we can rely on tools other than udunits to
handle more generally this new unit and the various CF conventions
calendars to convert coordinate values to other time units like "days
since ...".

Perhaps the biggest argument in favor of introducing a calendar_month
unit is that it should make it much easier for data providers to
generate correct time coordinates for data reported at monthly
intervals. Regardless of calendar, I think it is easy to generate
monthly time coordinates under the current CF standard that are simply
wrong. In contrast, everyone should be able to trivially create a
coordinate axis with values like (1, 2, 3, .... ) or (0.5, 1.5, 2.5,
...) without making a mistake.

best regards,
Karl






On 10/18/18 10:58 AM, Jonathan Gregory wrote:
Dear Martin

The definition of a calendar_month unit would also need rules about calendar
arithmetic e.g. What does 1 calendar_month since 31 January mean? What does
7.23 calendar_months since 31 January mean?

Best wishes

Jonathan


----- Forwarded message from Martin Juckes - UKRI STFC
<martin.juckes at stfc.ac.uk> <martin.juckes at stfc.ac.uk> -----


Date: Thu, 18 Oct 2018 16:33:28 +0000
From: Martin Juckes - UKRI STFC <martin.juckes at stfc.ac.uk>
<martin.juckes at stfc.ac.uk>
To: Jonathan Gregory <j.m.gregory at reading.ac.uk>
<j.m.gregory at reading.ac.uk>, "cf-metadata at cgd.ucar.edu"
<cf-metadata at cgd.ucar.edu>
    <cf-metadata at cgd.ucar.edu> <cf-metadata at cgd.ucar.edu>
Subject: Re: [CF-metadata] 'months since' and 'years since' time units

Dear Jonathan,


I think you could go further and disallow the use of "month" or "year"
as a time unit when the calendar is not standard.


If the "ncdump -t" option produces what the user expects when he has
units "months since 1900-01-01" and a 360 day calendar, then it is
going to be inconsistent with the current convention.


I still feel that there is an argument for enabling the storage of
information in user months in the files. E.g. I wish to compare
monthly mean data from 20 climate models which use a range of
different calendars. The mean across the models is not on any specific
calendar ... I could pretend it is and use units of "days since ...",
but the mappings from input time coordinates to output time
coordinates then become rather complex, when they should be trivial.
Having a "date" standard name which allowed the input data to have a
"calendar_month" coordinate would solve this (and I think Klaus's
suggestion would also solve it),


regards,

Martin

________________________________
From: CF-metadata <cf-metadata-bounces at cgd.ucar.edu>
<cf-metadata-bounces at cgd.ucar.edu> on behalf of Jonathan Gregory
<j.m.gregory at reading.ac.uk> <j.m.gregory at reading.ac.uk>
Sent: 18 October 2018 17:10:46
To: cf-metadata at cgd.ucar.edu
Subject: Re: [CF-metadata] 'months since' and 'years since' time units

Dear all

This is an interesting discussion, and I agree that's a tricky subject. If only
we could have a well-behaved Earth which orbited the sun in an integral and
easily factorisable number of days!

So far I still think that we should not change the way we interpret the units
string. It's in udunits format, and should be interpreted according to the
calendar attribute. I would suggest that it's helpful to regard time coords as
*encoded* and not necessarily easy for humans to read. It's certainly nice to
see "time=1, 2, 3, ..." months since a reference date - that is easy to read -
but when you get to 747 or 4689 months since a reference date, you don't know
what it means any more (unless you're extremely good at mental arithmetic), and
you might as well encode it as days.

The antidote to inconvenient encoding is convenient software. For example,
could cftime allow the user to construct a time coordinate variable with a
spacing of calendar months, but encode it in the netCDF file in days? Then it's
transparent. Similarly, time coordinate variables can be decoded into human-
readable strings by calendar-aware software. It seems to me that this isn't
different in principle from using ncdump to read a netCDF file, rather than
insisting it should be intelligible when read in hexadecimal. In fact, ncdump
itself has a -t option, which should help, according to the man page:

"-t controls display of time data, if stored in a variable that uses a udunits
compliant time representation such as `days since 1970-01-01' or `seconds since
2009-03-15 12:01:17' .... If this option is specified, time data values are
displayed as human-readable date-time strings rather than numerical values,
interpreted in terms of a `calendar' variable attribute, if specified. ...
Calendar attribute values interpreted with this option include the CF
Conventions values `gregorian' or `standard', `proleptic_gregorian', `noleap'
or `365_day', `all_leap' or `366_day', `360_day', and `julian'."

I agree with comments that if we introduced new units such as calendar_month
or 30day_month, people might well not use them, and would still be disappointed
that "month" wasn't what they expected.

The CF conformance document has a recommendation that "year" and "month" should
be used "with caution". I don't what the CF checker currently does with this.
We could change it to a recommendation that they should *not* be used, in which
case the checker would give a warning if they were.

Best wishes

Jonathan

----- Forwarded message from B?rring Lars <Lars.Barring at smhi.se>
<Lars.Barring at smhi.se> -----


Date: Thu, 18 Oct 2018 13:31:10 +0000
From: B?rring Lars <Lars.Barring at smhi.se> <Lars.Barring at smhi.se>
To: Martin Juckes - UKRI STFC <martin.juckes at stfc.ac.uk>
<martin.juckes at stfc.ac.uk>, David Blodgett
       <dblodgett at usgs.gov> <dblodgett at usgs.gov>, Ryan Abernathey
<ryan.abernathey at gmail.com> <ryan.abernathey at gmail.com>
Cc: "cf-metadata at cgd.ucar.edu" <cf-metadata at cgd.ucar.edu>
<cf-metadata at cgd.ucar.edu> <cf-metadata at cgd.ucar.edu>
Subject: Re: [CF-metadata] 'months since' and 'years since' time units

Dear Martin, David, all,

As Klaus points out, the aim of my suggestion is to make software
using CF aware of the fact that the unit "year" is different depending
on which calendar the model is implementing. To give an example:
If I want to know when the global average temperature has increased by
1.5C, or 4C, above pre-industrial time in the CMIP5 ensemble I will
get answers as a timedelta in days. As this is not really helpful I
might feel inclined to convert this to years, but now UDUNITS
definition of year is not helpful for those models having a 360_day or
365_day calendar. However, with the calendar-aware definition of year
such a calculation would be supported without having to deal with it
manually.

Now, on to the discussion about months. Before my previous post I
quickly read through extensive exchange on this list back in 2011, so
I really appreciate David's comment that it is a complex subject. And
that is the reason for why I suggested is always month is always a
year / 12. So, here is an attempt to summarise the suggestion in a
different way:

* standard and proleptic_gregorian calendars (status quo):
  o the number of days in a month is not an integer
  o same issue with respect to ordinary (western) world months.

* 365_day calendar:
  + the number of seconds in a month would change from being
"ill-defined (?)" as 84600 * 365.242198781 / 12 = 2574957.50141, to
more properly 84600 * 365 / 12 = 2573250
  o same issue with respect to ordinary (western) world months.

* 360_calendar:
  + the number of seconds in a month would change from being "very
ill-defined (?)" as 84600 * 365.242198781 / 12 = 2574957.50141, to
more properly 84600 * 360 / 12 = 2538000
  + the number of days in a month is an integer; 12 * 30 * 84600 = 2538000
  + the definition of a month is consistent with what is expected in
the "360_day world"
  o same issue with respect to ordinary (western) world months.

That is, even though the suggestion certainly do not solve everything
(of course!), the only argument against it, that I can see, is the
work to tease out the details and implement it in software packages.
As was extensively discussed in the 2011 threads, the real problem is
the varying length of the western world calendar months. But that is
the topic for another thread.


Kind regards,
Lars

________________________________
Fr?n: CF-metadata [cf-metadata-bounces at cgd.ucar.edu] f?r David
Blodgett [dblodgett at usgs.gov]
Skickat: den 18 oktober 2018 13:58
Till: Ryan Abernathey
Kopia: cf-metadata at cgd.ucar.edu
?mne: Re: [CF-metadata] 'months since' and 'years since' time units

Dear Ryan, All,

I hesitate to chime in on this thread as I know just how fraught this
topic can be, but then I think I know how fraught it can be so may
have something to offer. My experience is at the intersection of
climatological models and landscape models that are calibrated with
"real" data. I've worked with daily and monthly time series model
output and interpolated weather products that needs to match up to
observations but uses a noleap or 360 calendar. It's an enormous pain
and we as a community should do better. -- so the business case for
taking this complexity head on is there!

One resource I've found useful over the years is the [CDM
implementation](https://www.unidata.ucar.edu/software/thredds/current/netcdf-java/CDM/CalendarDateTime.html)
ut this does not
There are two factors at play.

1) Adding "calendar" to a udunits string avoids conversion to a number
of shorter time increments for long time increments (e.g. seconds per
month). It keeps things in the declared time units so you hit the
precise date boundaries you would expect.
2) The "calendar" attribute gets you to how to interpret the datum of
the time axis.

Especially relevant to this thread is:

  * uniform30day or 360_day = All years are 360 days divided into 30
day months.

With these two, I think the problems here are solved. However,
inevitably, people will omit the addition attribute for calendar or
fall back on normal "months since ..." when they actually mean
"calendar months since ..." and tell us 'why would you interpret my
data that way it makes no sense?!?' This is perfectly parallel to
spatial coordinates where people don't declare a datum for their
latitude/longidute coordinates. Without that information one can not
use the information with a level of precision that some use cases
require.

What I'm getting at is that CF should probably:
1) adopt enough attribute precision to fully describe what we are
trying to convey
2) make said attributes required or declare sensible defaults that
reduce ambiguity when users come knocking.

That said, I've had no success pushing the community to accept that
there should be a default lat/lon datum for software developers to go
on and I would not doubt that the same will be true here as ambiguity
and uncertainty is better than dead wrong in many cases. My stance is
that we should all be dead wrong for the same reason rather than each
implementor making an arbitrary decision so we all get different
answers (more ambiguity) from our software du-jour.

All the best,

Dave


On Oct 18, 2018, at 6:08 AM, Martin Juckes - UKRI STFC
<martin.juckes at stfc.ac.uk<mailto:martin.juckes at stfc.ac.uk>
<martin.juckes at stfc.ac.uk>> wrote:

Hello All,


I think the UNIDATA pull request referenced Jeff
(https://github.com/Unidata/cftime/pull/69) is mis-quoting the CF
Convention. As far as I can see, Unidata says that a month is exactly
one 12th of a year, and CF inherits this -- with the statement "For
similar reasons the unit month, which is defined in
udunits.dat<http://www.unidata.ucar.edu/software/udunits/>
<http://www.unidata.ucar.edu/software/udunits/> to be exactly year/12,
should also be used with caution."


I can't see the difference between Lars's suggestion and the status
quo. In UNIDATA a day is clearly defined as "period of time equal to
24 hours", which gives 84600 seconds.

regards,
Martin



________________________________
From: CF-metadata
<cf-metadata-bounces at cgd.ucar.edu<mailto:cf-metadata-bounces at cgd.ucar.edu>
<cf-metadata-bounces at cgd.ucar.edu>> on behalf of B?rring Lars
<Lars.Barring at smhi.se<mailto:Lars.Barring at smhi.se>
<Lars.Barring at smhi.se>>
Sent: 18 October 2018 09:29:50
To: Ryan Abernathey;
whitaker.jeffrey at gmail.com<mailto:whitaker.jeffrey at gmail.com>
<whitaker.jeffrey at gmail.com>;
cf-metadata at cgd.ucar.edu<mailto:cf-metadata at cgd.ucar.edu>
<cf-metadata at cgd.ucar.edu>
Subject: Re: [CF-metadata] 'months since' and 'years since' time units

Hi,

I have have come to think about this from a somewhat different
perspective. For some analyses, as well as when calculating certain
derived climatological statistics (aka climate indices), using
datasets based on different calendars the problem becomes obvious.

In the model world of a 365-day GCM one year _is_ 365 days, and in a
360_day GCM a year _is_ 360 days. In the case of a gregorian/standard
calendar GCM I am not sure whether it is 365.25 or 365.242198781 but
this is in practice less of a problem.

For datasets based non-standard calendars imposing the current UDUNITS
definition of a year leads to complications that require workarounds
if one is interested in for example the time elapsed until something
happens or the duration of some (long-lasting) events. One way to
partly mitigate these issues would be to use the time unit of
years_since_START or months_since_START, but this is warned against in
the CF Conventions and may software tools do not support it .

The fundamental issue is the inconsistency between the GCM year and
the UDUNITS year. So I would like to call on the wisdom of this list
to see whether the CF Convention could include a modification to the
definition of a year and month:

* standard calendar (no change)
1 day = 84600 seconds
1 year = 365.242198781 days
1 month = 365.242198781 / 12 days

* 365_day calendar
1 day = 84600 seconds
1 year = 365 days
1 month = 365 / 12 days

* 360_day calendar
1 day = 84600 seconds
1 year = 360 days
1 month = 360 / 12 days

That is, the seconds per day ratio is not changed thus maintaining the
consistency to other SI units. And, for the 360_day calendar month
follows the suggestion by Ryan and Jeffrey.


Kind regards,
Lars

--
Lars B?rring
FDr, Forskare
PhD, Research Scientist
SMHI  /  Swedish Meteorological and Hydrological Institute
Rossby Centre
SE - 601 76 NORRK?PING
Tel / Phone: +46 (0)11 495 8604
Fax: +46 (0)11 495 8001
Bes?ksadress / Visiting address: Folkborgsv?gen 17
________________________________
Fr?n: CF-metadata
[cf-metadata-bounces at cgd.ucar.edu<mailto:cf-metadata-bounces at cgd.ucar.edu>
<cf-metadata-bounces at cgd.ucar.edu>] f?r Ryan Abernathey
[ryan.abernathey at gmail.com<mailto:ryan.abernathey at gmail.com>
<ryan.abernathey at gmail.com>]
Skickat: den 17 oktober 2018 21:22
Till: whitaker.jeffrey at gmail.com<mailto:whitaker.jeffrey at gmail.com>
<whitaker.jeffrey at gmail.com>
Kopia: cf-metadata at cgd.ucar.edu<mailto:cf-metadata at cgd.ucar.edu>
<cf-metadata at cgd.ucar.edu>
?mne: Re: [CF-metadata] 'months since' and 'years since' time units
Hi everyone,
I am that user, and I'm new to this mailing list. Thank you all for
your work on CF conventions. It's such a valuable effort!
I want to note that this was inspired by the proliferation of datasets
in the wild that use "month" as their units. For example, nearly all
of the IRI Data Library does this, in conjunction with a 3"60_day"
calendar (example:
https://iridl.ldeo.columbia.edu/SOURCES/.NOAA/.NCEP-NCAR/.CDAS-1/.MONTHLY/.Diagnostic/.surface/.temp/).
My impression from talking to data providers is that no one is using
"360_day" calendar and "months" as units, and then expecting "months"
to be interpreted as 365.242198781/12 days. They all expect it to be
interpreted as 30 days. While there are various workarounds that can
be used at different levels of the software stack, the best solution,
IMHO, is to explicitly allow in CF conventions what Jeff proposed:
"months and years be interpreted as calendar months and years for
those calendars where they have a fixed length". I don't think this
will break existing applications.
Thanks,
Ryan
On Wed, Oct 17, 2018 at 3:06 PM Jeffrey Whitaker
<whitaker.jeffrey at gmail.com<mailto:whitaker.jeffrey at gmail.com>
<whitaker.jeffrey at gmail.com><mailto:whitaker.jeffrey at gmail.com>
<whitaker.jeffrey at gmail.com>> wrote:
Hi:  I'm a developer of the 'cftime' python package
(https://github.com/Unidata/cftime).  A user submitted a pull request
(https://github.com/Unidata/cftime/pull/69) that implements support
for a 30-day calendar month time unit for the '360_day' CF calendar.
Although using a 'month' time unit is a tricky proposition in general,
for this calendar it seems straightforward since every month has the
same length.  However, in the discussion of the pull request it was
pointed out that CF expects  that "the value of the units attribute is
a string that can be recognized by UNIDATA's Udunits package", and
that UDUNITS defines a month as 365.242198781/12 days.    My question
is this - is it reasonable for our python package to make an exception
to this rule for the 360_day calendar?  More generally, can months and
years be interpreted as calendar months and years for those calendars
where they have a fixed length, or will this deviate from the existing
CF conventions and break existing applications?
Regards, Jeff
--
Jeffrey S. Whitaker
NOAA/OAR/PSD  R/PSD1
325 Broadway, Boulder, CO, 80305-3328
Phone: (303)497-6313
FAX: (303)497-6449
_______________________________________________
CF-metadata mailing
listCF-metadata at cgd.ucar.edu<mailto:CF-metadata at cgd.ucar.edu>
<CF-metadata at cgd.ucar.edu><mailto:CF-metadata at cgd.ucar.edu>
<CF-metadata at cgd.ucar.edu>http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
_______________________________________________
CF-metadata mailing
listCF-metadata at cgd.ucar.edu<mailto:CF-metadata at cgd.ucar.edu>
<CF-metadata at cgd.ucar.edu>http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
_______________________________________________
CF-metadata mailing
listCF-metadata at cgd.ucar.eduhttp://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
----- End forwarded message -----
_______________________________________________
CF-metadata mailing
listCF-metadata at cgd.ucar.eduhttp://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
----- End forwarded message -----
_______________________________________________
CF-metadata mailing
listCF-metadata at cgd.ucar.eduhttp://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
_______________________________________________
CF-metadata mailing
listCF-metadata at cgd.ucar.eduhttp://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
_______________________________________________
CF-metadata mailing
listCF-metadata at cgd.ucar.eduhttp://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
_______________________________________________
CF-metadata mailing
listCF-metadata at cgd.ucar.eduhttp://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
-- 
[image: CICS-NC] <http://www.cicsnc.org/> Visit us on
Facebook <http://www.facebook.com/cicsnc> *Jim Biard*
*Research Scholar*
Cooperative Institute for Climate and Satellites NC <http://cicsnc.org/>
North Carolina State University <http://ncsu.edu/>
NOAA National Centers for Environmental Information <http://ncdc.noaa.gov/>
*formerly NOAA?s National Climatic Data Center*
151 Patton Ave, Asheville, NC 28801
e: jbiard at cicsnc.org
o: +1 828 271 4900
*Connect with us on Facebook for climate
<https://www.facebook.com/NOAANCEIclimate> and ocean and geophysics
<https://www.facebook.com/NOAANCEIoceangeo> information, and follow us on
Twitter at _at_NOAANCEIclimate <https://twitter.com/NOAANCEIclimate> and
_at_NOAANCEIocngeo <https://twitter.com/NOAANCEIocngeo>. *
_______________________________________________
CF-metadata mailing list
CF-metadata at cgd.ucar.edu
http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.cgd.ucar.edu/pipermail/cf-metadata/attachments/20181019/aec8c448/attachment-0001.html>

Received on Fri Oct 19 2018 - 13:53:13 BST