datetime-design-pattern

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This page is a draft.

Datetime Design Pattern

This is a page for exploring a datetime design pattern.

Purpose

Practical Need

  • This design pattern arose as a result of solving the practical need for human readable dates for hCalendar.

How to use it

  • enclose the human-friendly datetime that you want to make machine readable with <abbr>
  • as per the class-design-pattern, add the appropriate class attribute to the abbr element
  • add a title attribute to the abbr element with the machine readable ISO8601 datetime or date as the value

Current uses

The pattern, which is now available as part of hCalendar, hCard and hReview, is: 

<abbr class="foo" title="YYYY-MM-DDTHH:MM:SS+ZZ:ZZ">Date Time</abbr>

where foo is the semantic classname which is being applied to this date/time, the title of the <abbr> is an ISO 8601 date/time, with an appropriate level of specificity, and "Date Time" is a human-friendly representation of the same date/time.

An alternative, if you are using UTC-based timestamps, would be: 

<abbr class="foo" title="YYYY-MM-DDTHH:MM:SSZ">Date Time</abbr>

with a single "Z" as per ISO 8601

Profile of ISO8601

We recommend that any microformat using the date-time-design pattern use a profile of ISO8601. There are currently two widely used profiles which SHOULD be reused.

Accessibility issues

Note: There are some accessibility issues [1] with Datetime Design Pattern, and conerns that its use could breach WCAG acessibility guidelines, that are being addressed as part of the abbr-design-pattern-issues discussion. Possible change recommendations may follow after the accessibility testing is complete.

Discussion

This pattern is likely to be highly resuable.

--User:RyanKing

Can this not be viewed as a microformat in itself?

--User:DimitriGlazkov

It could, but inventing a microformat for the sake of inventing a microformat is against the microformat principles. If there is a specific real world problem (and uses cases) that such an elemental microformat would solve, then it would be worth considering.

Until then it is best to keep the <abbr> datetime concept merely as a microformat design pattern, to be used in _actual_ microformats that have a demonstrated practical need.

-- Tantek

Excerpt from #microformats Aug 18th. Please edit! 

Aug 18 15:16:14 <Tantek>	DanC, what do you think of RFC3339?
Aug 18 15:17:14 <Tantek>	ISO8601 subset
Aug 18 15:17:19 <DanC>	        Date and Time on the Internet: Timestamps http://www.ietf.org/rfc/rfc3339.txt
Aug 18 15:17:30 <DanC>	        Klyne is a good guy. I wonder if I talked with him about this.
Aug 18 15:17:32 <Tantek>	compat with W3C-NOTE-DATETIME
Aug 18 15:17:50 <Tantek>	compat with xsd:dateTime
Aug 18 15:17:57 <Tantek>	it's a strict intersection subset
Aug 18 15:17:59 <DanC>	        I consider W3C-NOTE-DATETIME obsoleted by XML Schema datatype-- yeah.. xsd:dateTime
Aug 18 15:18:32 <Tantek>	compare/contrast normatively using xsd:dateTime vs. RFC3339
Aug 18 15:18:41 <Tantek>	note: Atom 1.0 chose RFC3339
Aug 18 15:18:50 <Tantek>	i would like input from the microformats community on this
Aug 18 15:19:27 <DanC>	        in what context are you evaluating RFC 3339?
Aug 18 15:19:28 <jcgregorio>	http://bitworking.org/news/Date_Constructs_in_the_Atom_Syndication_Format
Aug 18 15:21:24 <DanC>	        which microformat is the question coming from, Tantek ?
Aug 18 15:23:31 <DanC>	        "   The grammar element time-second may have the value "60" at the end of
Aug 18 15:23:31 <DanC>	        months in which a leap second occurs" The XML Schema WG is in the 27th level of
                                leap-second-hell for the past few months, I gather.
Aug 18 15:24:21 <DanC>	        yeah... here's the scary bit: "   Leap seconds cannot be predicted far into the
                                future.  The
Aug 18 15:24:21 <DanC>	        International Earth Rotation Service publishes bulletins [IERS] that
Aug 18 15:24:21 <DanC>	        announce leap seconds with a few weeks' warning."
Aug 18 15:26:03 <Tantek>	DanC, which microformats? any/all that use datetime fields.
Aug 18 15:26:36 <DanC>	        hard to give useful advice, then.
Aug 18 15:26:58 <DanC>	        I expect they'll use datetime fields for different things that have different
                                cost/benefit trade-offs
Aug 18 15:27:26 <DanC>	        do you know of any particular differences that matter to anybody?
Aug 18 15:56:43 <KragenSitaker>	RFC3339 suggests -07:00, which seems like an improvement over -0700 anyway
Aug 18 15:56:49 <Tantek>	Kragen, agreed
Aug 18 15:57:01 <Tantek>	RFC3339 is certainly preferable to the ISO8601 subset in iCalendar
Aug 18 16:05:57 <DanC>	        Tantek's right, Kragen; iCalendar looks like it solves the local timezone
                                problem but doesn't.
Aug 18 16:06:14 <DanC>	        and it's true that there's no standard solution to the local timezone problem
Aug 18 16:06:39 <Tantek>	so instead of appearing to solve the problem but not solving it, we chose to
                                provide the ability to *approximate* the local timezone using e.g. "-07:00"
Aug 18 16:06:49 <DanC>	        the simplest thing is to have people use Z time in hCalendar. But I gather
                                that's unacceptably unusable?
Aug 18 16:07:35 <Tantek>	DanC, yes, the simplest thing is to have everyone use UTC Z
Aug 18 16:07:38 <Tantek>	However
Aug 18 16:07:50 <Tantek>	it is not *nearly* as usuable/verifiable
Aug 18 16:07:55 <Tantek>	as -07:00 etc.
Aug 18 16:08:02 <Tantek>	hence the decision to go with the latter
Aug 18 16:08:12 <Tantek>	some degree of human verifiability is important here
Aug 18 16:14:21 <Tantek>	DanC, my perception is that RFC3339 is a subset
Aug 18 16:17:00 <DanC>	        time-numoffset  = ("+" / "-") time-hour ":" time-minute
Aug 18 16:17:34 <DanC>	        ok, then I can't see any differences. (modulo recent leap seconds issues that
                                may affect xsd:dateTime )
Aug 18 16:18:07 <Tantek>	would be interesting to know why Atom 1.0 chose RFC3339 over xsd:dateTime
Aug 18 16:18:21 <Tantek>	if there was a "real" reason or if it was arbitrary / coin-flip.

Here's an exhaustive comparison from ndw. I think xsd:dateTime also allows unqualified local times, while RFC3339 allows only UTC with no known timezone (-00:00). In the end, Atompub followed the advice of Sam Ruby and Scott Hollenbeck, our area director. Atom dates make some additional restrictions on RFC3339, such as uppercase T and Z characters for compatibility with xsd:dateTime, RFC3339, W3C-DTF, and ISO8601. --Robert Sayre 

Aug 18 16:18:43 <KragenSitaker>	rfc3339 is pretty short.
Aug 18 16:19:36 <Tantek>	DanC, BTW, which came first? REC for xsd:dateTime or RFC3339?
Aug 18 16:19:50 <DanC>	        RFC3339 is dated July 2002 ...
Aug 18 16:19:54 <KragenSitaker>	Right --- and you might be able to understand xsd:dateTime without
                                reading all of xml schema, you wouldn't be confident of it
Aug 18 16:20:25 <DanC>	        W3C Recommendation 28 October 2004 ... but that's 2nd ed...
Aug 18 16:20:47 <DanC>	        W3C Recommendation 02 May 2001
Aug 18 16:22:10 <DanC>	        I don't see a BNF in http://www.w3.org/TR/2001/REC-xmlschema-2-20010502/#dateTime ...
Aug 18 16:22:43 <KragenSitaker>	yeah, appendix D of the current xml schema datatypes document seems
                                a little scanty, actually
Aug 18 16:23:28 <DanC>	        ah... 2nd ed of http://www.w3.org/TR/xmlschema-2/#date is much more
                                explicit about syntax.
Aug 18 16:23:30 <KragenSitaker>	it's 1100 words but still doesn't give any examples
Aug 18 16:23:35 <DanC>	        still, it's given in prose and not BNF
Aug 18 16:24:17 <KragenSitaker>	sections 3.2.9 through 3.2.14 seem to be the relevant ones around #date
Aug 18 16:24:29 <KragenSitaker>	which is another 2200 words
Aug 18 16:24:42 <DanC>	        wow... they changed the canonical form of date from always-Z to
                                timezone-allowed between 1st edition and 2nd edition
Aug 18 16:25:01 <Tantek>	Kragen, DanC, these are very good analyses
Aug 18 16:25:21 <Tantek>	could I ask you to summarize the pros/cons for each in a new section at
                                end of http://microformats.org/wiki/datetime-design-pattern
Aug 18 16:25:22 <Tantek>	?
Aug 18 16:25:58 <KragenSitaker>	rfc 3339 is 4000 words, excluding the last two pages of boilerplate.
Aug 18 16:26:31 <KragenSitaker>	so it's actually longer than the datetime-relevant parts of XSD but it
                                seems much more rigorous and clear
Aug 18 16:28:37 <DanC>	        my advice is: normatively cite both, and claim they specify the same
                                syntax, and let anybody who discovers otherwise send you a bug report
                                with a test case
Aug 18 16:29:12 <KragenSitaker>	danc: nice hack


The RFC3339 has a mandatory TIME portion of the DATE-TIME. Some vCard/iCalendar DATE-TIME stamps can omit the TIME. For instance, DTSTART, if that is a full day event, then you can omit the time. BDAY in vCard can be respresented by only a DATE. I like the idea of restricting the possible date formats, but i think that TIME should be optional, which it isn't in RFC3339. - brian suda

RFC 3339 allows lowercase 't' and 'z' while XSD doesn't. Specifying RFC 3339 plus 'T' and 'Z' MUST be caps will make them the same. - Joe Gregorio

---

A few questions: asked by CharlesBelov 16:57, 24 Apr 2007 (PDT), answered by JamesCraig on 15:58, 5 Jul 2007 (PDT).

  1. Would it make more sense for documenting the alternative codings pitting the abbr tag vs. other tags to be on this page? Answer: That documentation should go on the assistive-technology-abbr-results page.
  2. Would using the title attribute of the abbr tag to encode the machine-readable date in fact cause a failure of WCAG 2.0 Accessibility? What about USA Section 508? It does appear to violate Technique for WCAG 2.0 H28: Providing definitions for abbreviations by using the abbr and acronym elements, although that is a supporting document and does not have the force of a guideline. Answer: Yes, it appears that is in violation of WCAG, 508, et al, so alternatives are being discussed on the assistive-technology-abbr-results page.
  3. In order to maintain accessibility, would it make sense to enclose the machine-readable date in a span with a style of "display:none" instead of using the abbr tag? Answer: please refer to and add any suggestions to assistive-technology-abbr-results.
  4. For that matter, wouldn't you want to style such an abbr tag with text-decoration:none to hide that an abbr tag was used? Otherwise, visitors might cursor over the time, see the machine time, and be annoyed that their time was wasted or else be confused. And I don't think you can suppress the title from coming up if the human-readable time was inadvertently hovered. Answer: Microformats should not rely on CSS in order to work properly, but again, that discussion can be found here: assistive-technology-abbr-results.

Code

The following regular expression (parsed VERBOSE) should break apart a datetime and cover many lightly broken cases seen in the wild. This has been tested under Python. 

 ^
 (?P<year>\d\d\d\d)
 ([-])?(?P<month>\d\d)
 ([-])?(?P<day>\d\d)
 (
  (T|\s+)
  (?P<hour>\d\d)
  (
   ([:])?(?P<minute>\d\d)
   (
    ([:])?(?P<second>\d\d)
    (
     ([.])?(?P<fraction>\d+)
    )?
   )?
  )?
 )?
 (
  (?P<tzzulu>Z)
  |
  (?P<tzoffset>[-+])
  (?P<tzhour>\d\d)
  ([:])?(?P<tzminute>\d\d)
 )?
 $

See Also

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