Chris,
I see what you are saying about category, yet it is metric. Within the
domain of months and years (without days, leap days, etc), it is
completely possible to do exact math, just as we can within the domain
of seconds, minutes, hours, and days. The problem arises when you try to
bridge between these two domains. The current UDUNITS tries to keep it
all in one domain, with the result that values in months and years
become problematic.
Grace and peace,
Jim
On 10/19/18 3:53 PM, Chris Barker - NOAA Federal wrote:
> 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
> <mailto: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> 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 calend
ar_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] för 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 "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> -----
>>>>>
>>>>>> Date: Thu, 18 Oct 2018 16:33:28 +0000
>>>>>> From: Martin Juckes - UKRI STFC<martin.juckes at stfc.ac.uk>
>>>>>> To: Jonathan Gregory<j.m.gregory at reading.ac.uk>,"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> on behalf of Jonathan Gregory<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> -----
>>>>>>
>>>>>>> Date: Thu, 18 Oct 2018 13:31:10 +0000
>>>>>>> From: B?rring Lars<Lars.Barring at smhi.se>
>>>>>>> To: Martin Juckes - UKRI STFC<martin.juckes at stfc.ac.uk>, David Blodgett
>>>>>>> <dblodgett at usgs.gov>, Ryan Abernathey<ryan.abernathey at gmail.com>
>>>>>>> Cc:"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>> 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/> 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>> on behalf of B?rring Lars <Lars.Barring at smhi.se<mailto: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>;cf-metadata at cgd.ucar.edu<mailto: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>] f?r Ryan Abernathey [ryan.abernathey at gmail.com<mailto:ryan.abernathey at gmail.com>]
>>>>>>> Skickat: den 17 oktober 2018 21:22
>>>>>>> Till:whitaker.jeffrey at gmail.com<mailto:whitaker.jeffrey at gmail.com>
>>>>>>> Kopia:cf-metadata at cgd.ucar.edu<mailto: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><mailto: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
>>>>>>>
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