<time> elements are invalid

Here's an example from http://vim-scripts.org/news/2011/06/12/rss-gone.html

```
 <time class="updated" pubdate="" datetime="23 Jun 2011">23 Jun 2011</time>
```

The datetime attribute needs to be formatted like this: http://www.whatwg.org/specs/web-apps/current-work/multipage/common-microsyntaxes.html#valid-global-date-and-time-string
