(Article updated to correct some typos noticed by commenters, and clarify some aspects.)
Avid HTML5 watchers will know that the
<time> element was dropped from HTML, then re-instated, with more New! Improved! semantics.
As before, you can put anything you like between the opening and closing tags – that’s the human-readable bit. The machine-readable bit is contained within a
datetime attribute. Dates are expressed YYYY-MM-DD.
Previously, you could only mark up precise dates. So, 13 November 1905 could be expressed in HTML
<time datetime="1905-11-13"> but November 1905 couldn’t be. This is a problem for historians where sometimes the precise date isn’t known.
Now, “fuzzy dates” are possible:
<time datetime="1905">means the year 1905
<time datetime="1905-11">means November 1905
<time datetime="11-13">means 13 November (any year)
<time datetime="1905-W21">means week 21 of 1905
As before, times are expressed using the 24 hour clock. Now, you can separate the date and time with a space rather than a “T” (but you don’t have to). So both of these are valid:
<time datetime="1905-11-13 09:00">
You can localise times, as before. Appending “Z” to the timezone indicates UTC (a way of saying “GMT” without it being comprehensible to normal people). Otherwise, use an offset:
<time datetime="09:00Z">is 9am, UTC.
<time datetime="09:00-05:00">is 9am in the timezone 5 hours behind UTC.
<time datetime="09:00+05:45">is 9am in Nepal, which is UTC + 5 hours and 45 minutes.
In New! Improved! HTML5 <time>, you can represent durations, with the prefix “P” for “period”.
The datetime attribute “D” for days, “H” for hours, “M” for minutes and “XQ” for seconds. Just kidding – that’s “S”.
You can separate them with spaces (but you don’t have to). So
<time datetime="P4D"> is a duration of 4 days, as is
<time datetime="P 4 D">.
Using a “T” after the “P” marker allows you to be more precise:
<time datetime="PT23H 9M 2.343S"> is a duration of 23 hours, 9 minutes and 2.345 seconds.
Alternatively, you can use a duration time component.
Whichever you choose, it’s represented internally as a number of seconds. Because of this, you can’t specify a duration in terms of months, because a month isn’t a precise number of seconds; a month can last from 28 to 31 days. Similarly, a year isn’t a precise number of seconds; it’s 12 months and February sometimes has an extra day.
You still can’t represent dates before the Christian era, as years can’t be negative. Neither can you indicate date ranges. To mark up From “21/02/2012 to 25/02/2012″, use two separate <time> elements.
The pubdate attribute (a boolean that indicates that this particular date is the publication date of the parent <article> (or, if there is none, the whole document) is currently missing from the W3C version of the spec, but is still current in the WHATWG version. Its status is unclear to me (but I’m still using it).
Update 10 August 2012: in response to a query, I checked again and
pubdate is gone from both the WHATWG and W3C specs.