Thanks for both these enlightening comments.

To try to clarify the position as it appears to me, now, hoping that it might be generally helpful...

We have "gregorian" because there is a consensus about what Gregorian dates mean.
We don't appear (yet) to have a full consensus on the interpretation of any other calendars.

If there were in the future an agreement on how to name different interpretations or versions of e.g. Julian calendars, these could be added in to the list, either as a single agreed interpretation, or as a set of variants from various ages, primarily as used to record the time of current events (retrospective attribution always being more tricky.)

LIkewise for any other well-defined calendar system. Ideally, from a linked data point of view, every separate calendar-interpretation would have its own URI. :-)

One challenge is that the issue of cross-mapping different calendars is not of interest only to us, so maybe it is not for us to settle either?

Meanwhile, I don't see the force of an objection that dates alone do not reliably convert. Naturally, one person's day may start at a different time to another person's. Isn't what matters whether there was a community that once agreed on a calendar convention? Given that, it might be possible in principle, though often not in practice, to cross map to other calendars in some way, though not necessarily agreeing on day or year boundaries. At least the proposed spec offers the means of representing the uncertainty that results...


On 8 November 2010 16:54, ashtongj <[log in to unmask]> wrote:
If a provision is added to specify the Julian calendar, it will be necessary to distinguish the Julian calendar as observed in Rome since 45 BC versus the Julian calendar as extrapolated backward from the middle ages. They did not agree until around AD 8.

Gerry Ashton

On 2010-11-08 11:46 AM, Edward C. Zimmermann wrote:
The names are, I think, the least problem--- we can just define a controlled
The biggest problem is that calendars such as the Jewish/Hebrew don't have
reversible transformations into Gregorian or Julian unless time and day of the
week are also specified--- from which one could determine if its before
sundown at the place (and, in fact, the same location may have two different
representations for two different people as there are sometimes multiple
customs that might be called upon to define sundown).
Keeping to the Jewish/Hebrew calendar (and the Babylonian and other Lunisolar
calendars as well) we also can't resolve completely against our min/sec model.
The Jewish hour is divided into 1080 units: 72 units per degree celestial
rotation or 360 * 72/ 24. Going back to Hebrew/Jewish now from a transformed
date, however, won't work since the Julian/Gergorian day ends at Midnight and
there is no way to determine the date. Its roughly based upon the stars
viewable from the location--- with good visibility--
and so these is insufficient information in a date/time to go back.

On Mon, 8 Nov 2010 10:06:48 -0500, Denenberg, Ray wrote
From: Discussion of the Developing Date/Time Standards
[mailto:[log in to unmask]] On Behalf Of Simon Grant
Sent: Saturday, November 06, 2010 8:13 AM
To: [log in to unmask]
Subject: Re: [DATETIME] Extended Date/Time Format DRAFT specification for
review through December 6
"3. Miscellaneous / Calendar
It seems a no-brainer to define "julian" as an alternative to "gregorian",
though other calendars would presumably be more difficult, as they may have
different components. At least, in my limited understanding, Julian and
Gregorian calendars have the same components."

I took my cue here from the draft VCard specification,, and
also the draft HTML5 spec (at least the WHATWG version if not WC) calendar
parameter (calscale), in which they define only the single value, 'gregorian'.

My thinking is, I am hesitant to specify any values for calendars at all, as
I am unaware of any established vocabulary for calendar names (however, if
there is one, I would be interested in revisiting this issue). I am not
terribly reluctant to specify 'gregorian' because the vCard specification uses
that term, and even though that's a draft spec, if's a safe bet that it won't
change. I'm sure if's a safe bet that when Julian calendars are accomodated
the term 'julian' will be used. But there may also be good reasons why vCard
and the HTML people didn't want to include it now, perhaps because if they did
then the spec would need to detail the differences.

Anyway this is open for discussion.



 Edward C. Zimmermann, NONMONOTONIC LAB
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Simon Grant
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