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Time element

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Revision as of 17:13, 6 August 2010 by Tantek (talk | contribs) (restored second level headings for proposals because 3rd (and lower) level headings made them run together (presentation / usability / readability), grammar)
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Research, data, use cases, issues, and enhancements related to the HTML5 time element.

Syntax improvements for reducing DRY violations

composite nested time elements

A time element should permit child time elements which may contain only partial date time information which can then be composed into more complete date time information.

This is intended as a cleaner way to provide functionality equivalent to the microformats value-class-pattern date and time values pattern.


Currently the <time> element forces you to duplicate and hide date time information if you want to avoid displaying the not-very-friendly full ISO datetime:

<p class="vevent">
  <span class="summary">I went to the cafe</span> at 
  <time class="dtstart"
        datetime="2010-08-05T18:00:00">18:00 on 2010-08-05</time>.

Note the date and time information is duplicated (violating DRY, placing the content at risk of divergence).

microformats value class pattern DRY advantage

With the microformats value-class-pattern date and time values pattern you could instead mark this up like this:

<p class="vevent">
  <span class="summary">I went to the cafe</span> at 
  <span class="dtstart">
     <span class="value">18:00</span> on 
     <span class="value">2010-08-05</span>

Advantages: no duplication of time and date data! (avoiding DRY violation) If you need to update the info, you only have to update it in one place, thus reducing the chances of inforot.

Disadvantage: the loss of the HTML5 time semantic.

simple nested time example improvement

We'd like to have our time and date time separation as well, so this should work:

<p class="vevent">
  <span class="summary">I went to the cafe</span> at 
  <time class="dtstart">
     <time>18:00</time> on 

summary of updated datetime algorithm

In short: the algorithm for determining the "datetime" of a time element should:

  1. check for an explicit 'datetime' attribute (allowing a local to element override regardless of child elements)
  2. check for nested <time> elements, and if any are found, compose their values into a more complete date and time (use the first date found if any, then the first time found, if any. thus latter dates or times are gracefully ignored)
  3. use the complete contents of the <time> element as its datetime value.

Essentially, step 2 is added to enable composing nested child time elements.

applicability to microdata

All of the aforementioned advantages for microformats apply to microdata use of the <time> element as well. microformats are used in the above examples as that is the type of content (including the value class pattern) that is being published today (e.g. see http://microformats.org/wiki/events - the markup on that page itself).

nested time example with datetime attribute

If the publisher prefers to publish a "localized" form of dates (rather than the previous simple example with the most overall internationally human-friendly/readable YYYY-MM-DD ISODate), they can still do so:

<p class="vevent">
  <span class="summary">I went to the cafe</span> at 
  <time class="dtstart">
     <time>18:00</time> on 
     <time datetime="2010-08-05">August 5th, 2010</time>

Advantage: The advantage here over the current time element is that the DRY violation is limited to only the date information (instead of date and time information), thus reducing the risk of data divergence due to duplication.

nested time example with two datetimes

If the publisher prefers to publish a "localized" form of times, they can do that as well:

<p class="vevent">
  <span class="summary">I went to the cafe</span> at 
  <time class="dtstart">
     <time datetime="18:00:00">6pm</time> on 
     <time datetime="2010-08-05">August 5th, 2010</time>

Advantage: The two separate 'datetime' attributes (containing just the time and just the date) are more human-readable than a single datetime attribute containing both, and thus there is a slightly better chance that the few humans that check would correctly determine whether the times and dates in the datetime attributes represent the same value as the content of the element.

The AM/PM proposal below further helps improve this example.

nested time discussion

Opinions / discussion:

  • +1 Tantek - I'd really like to be able to more cleanly markup dates and times than the best we have been able to do so far with microformats (the aforementioned value-class-pattern), and HTML5 presents us with the potential to do so.
  • -1 Andy Mabbett - Introduces excessive complexity on the apparent assumption that a significant proportion of dates in the wild (or even in microformats in the wild) use the format "2010-08-05" and not more human-readable and accessible prose such as, say, "5 August 2010" or "August 5th, 2010". No evidence (also supposedly required by the microformats "process") has been provided to show that this is the case. {If the apparent assumption is not made, then this fails 80/20.)
  • ...

am pm and coarser time parsing

Right now time values inside a <time> element are required to specify hours in 24 hour time.

In our experience with the microformats value class pattern date and time values we've found it is relatively easy to both specify and implement (multiple implementations) parsing of (potentially coarser) am and pm values to permit a broader set of values to marked up directly (rather than with a separate datetime/title attribute).

In short, the current <time> element only allows for the following time syntax:

  • HH:MM:SS - where HH is in 24 hour time.

am pm syntax summary

This proposal expands the allowed time syntax to:

  • HH:MM:SSam
  • HH:MM:SSpm
  • HH:MMam
  • HH:MMpm
  • HHam
  • HHpm

am pm syntax details

  • periods, whitespace, case-insensitivity. "am" and "pm" mean "am or a.m." and "pm or p.m." with optional leading ("6 pm") and intermittent ("6 p. m.")white-space; and are case-insensitive ("6 PM").
  • implied 00 minutes and seconds. When :SS or :MM:SS is omitted, infer :00 or :00:00, respectively.
  • handling of HH of 12. for "am" values, HH of "12" is treated as "0". for "pm" values, HH of "12" is treated as "12".

simple am pm example

A simple example:

I went to the cafe at <time>6pm</time>.

Advantage: by specifying am and pm times that can be parsed directly from the contents of the

am pm example with nested time elements

Example (uses aforementioned composite nested time element proposal as well)

<p class="vevent">
  <span class="summary">I went to the cafe</span> at 
  <time class="dtstart">
     <time>6pm</time> on 

Advantage: again, this reduces DRY violations, in this case further improving upon the composite nested time elements case.

am pm discussion

Opinions / discussion:

  • +1 Tantek - in practice we in the microformats community have found that enabling users to markup am/pm times leads to many more cases where we can avoid violating DRY and thus encourage greater accuracy over time for such content. I think the HTML5 <time> element presents us with the opportunity to more cleanly markup times (than what we've been able to do with the aforementioned microformats value-class-pattern), and thus we should do so.
  • 0(query) Andy Mabbett - see above for concerns over date formatting.
    • queries moved to am pm FAQ section with answers. - Tantek 16:57, 6 August 2010 (UTC)
  • ...

am pm FAQ

noon and midnight

Question: How does this cater for "noon" and "midnight"?

Answer: This proposal does not address the (English) language specific terms of "noon" and "midnight".

ambiguity over 12am and 12pm

Question: How does this cater for the ambiguity over "12am" and "12pm"?

Answer: Proposal clarified to explicitly treat 12am as 00:00:00, and 12pm as 12:00:00.

am pm i18n

Question: How does it internationalise?

Answer: am and pm are (Latin origin) terms that are "used informally in most of the world."[1]

Date granularity

year only

The time element should accept just a year.

ISO8601 syntax
use case research

Opinions / discussion:

year month only

The time element should accept just a year and a month.

ISO8601 syntax
use cases
Blog/publishing archive pages - see Benward.me, ablognotlimited.com (need specific links to archive pages), http://www.flickr.com/photos/tantek/archives/
output equivalent of <input type="month">, see impedance match new date time inputs above.

Opinions / discussion:

year week only

The time element should accept just a year and a week number.

ISO8601 syntax
use case research
no examples in the wild currently. If anyone knows of any sites which publish references to specific weeks of a year, either by name / expression (e.g. "first week of the year") or by specific number (e.g. "weeks 1-26"), please provide URLs and quotes of example content.
output equivalent of <input type="week">, see impedance match new date time inputs above.
to provide the output equivalent of <input type="week">

See impedance match new date time inputs above.

Opinions / discussion:

  • +1 Tantek per good design of impedance matching date time inputs.
  • ...

month day only

The time element should accept just a month and a day.

ISO8601 syntax
use case research

Opinions / discussion:

Fuzzy dates

The time element should accept fuzzy (uncertain, approximate) dates ("around 18 June 1855" "summer 1970", "circa December 1963", "flourished 1580"), centuries, and eras ("Edwardian", "bronze age", "Jurassic")".

Opinions / discussion:

  • +1 Andy Mabbett (Per use cases in "Extended Date Time Format" proposals & TEI - see external links)
    • Uncertainty possibly resolved by a "certainty" attribute:
      <time datetime="1855-06-18" certainty="3days">around 18 June 1855</time>
      <time datetime="1970-06" certainty="45days">summer 1970</time>
      (with "45days" meaning "+/- 45 days" - in other words, a 90-day window, and similar allowance for year or other ranges; or:
      <time datetime="1963-12" certainty="circa">circa December 1963</time>
      with pre-defined prose values allowed, such as "flourished", "notbefore", "notafter", etc.
  • +1 Jim O'Donnell (Dates such as 'circa 1910' published on Flickr eg. The RNVR Training Ship 'Buzzard'… also a list of fuzzy dates for a set of photos.)
  • -1 Tantek - I'm opposed to an abstract proposal without a precise syntax to evaluate that would solve the use cases / examples listed. I'm open to changing my opinion on this (to 0 or +1 depending on the simplicity/impact) if a more precise/complete syntax is provided.

HTML5 internal consistency

impedance match new date time inputs

The time element should be able to represent every granularity of times and dates that the new date time <input> elements allow. Here is a list of all the date time <input> elements along with the corresponding <time> element usage (if applicable)

<input type="date">           - <time>YYYY-MM-DD</time>
<input type="datetime">       - <time>YYYY-MM-DDTHH:MM:SS</time>
<input type="month">          - not supported in current time element
<input type="week">           - not supported in current time element
<input type="time">           - <time>HH:MM:SS</time>
<input type="datetime-local"> - <time>HH:MM:SS-ZZ:YY</time>
New proposed input elements:
<input type="year">           - not supported in current time element
<input type="month-day">      - not supported in current time element

In particular the <time> element is missing support for the following date inputs:

In addition, if the new proposed input elements are accepted, the respective time element support should be added as well:

Opinions / discussion:

Miscellaneous proposals

Calendar scale

The time element should accept a calendar scale (CALSCALE; default is GREGORIAN) per (and to facilitate interoperability with) the emergent vCard 4 specification, to allow for the the mark-up of non-Gregorian (e.g. Julian) dates, using one of a set of pre-defined CALSCALE types.


<time datetime="1330-06-01" calscale="julian">1 June 1330</time>

User agents could be instructed to ignore any unrecognised CALSCALE value, treating the contents of the element as plain text for data-processing (but not styling) purposes. This would prevent, for example the processing of the above example by an agent written to deal only with Gregorian dates. (At some point, CSS should recognise CALSCAlE, allowing authors to, say, style all Julian dates differently to Gregorian dates.)

Opinions / discussion:

  • +1 Andy Mabbett (Per use cases in VCARDDAV, EDTF & TEI - see external links)
  • -1 Tantek - Despite years of the presence of the CALSCALE feature in iCalendar etc., there are no implementations (AFAIK) of non-GREGORIAN CALSCALE values in iCalendar etc. user agents, thus there is no reason to believe that specifying it in HTML5 would actually encourage any other user agents to implement it either.

Choose different default date

The statement that valueAsDate IDL attribute should return the value 1970-01-01 plus the appropriate time when the time element contains no date creates a problem that there are likely to be time elements that explicitly contain that date.

A better choice would be a value that is highly unlikely to be encountered, and would be implausible as an actual date in most applications, perhaps 9999-12-31.

Opinions / discussion:

  • 0 (comment) Andy Mabbett - 9999-12-31 may well occur in real applications (projected comet sightings, say). Can we return either an invalid date (perhaps 9999-02-31) or an error code?
  • -1 Tantek - I don't see any other default date as being significantly different.
  • ...

Issues without specific proposals

Specification ambiguities

The specification requires that time be expressed as UTC (or another time zone with a specified offset from UTC). However, the representation of leap seconds is not specified. Further, the algorithms to convert between string and number are flawed, because the number is described as "number of milliseconds elapsed from midnight UTC on the morning of 1970-01-01" but the actual number of milliseconds includes all kinds of strange decisecond offsets during the period 1961-01-01 to 1972-01-01. Also, UTC did not exist before about 1960.

Unix timekeeping has a long history of terrible definitions, and Unix notions of time should be totally rejected and expunged.

External links


Blog posts, Twitter updates etc. may be tagged HTML5time or #HTML5time

Prior discussion