If the datetime attribute includes a complete time and the pubdate attribute like this:

<time datetime="2010-11-15T00:00:00" pubdate>foo</time>

The error message will be:

<blockquote>
Error: Attribute pubdate not allowed on element time at this point.

From line n, column n; to line n, column n

<time datetime="2010-11-15T00:00:00" pubdate>foo</time>
</blockquote>

Adding at timezone identifier removes the error message.

Switching the order of datetime and pubdate will give me ca correct error message.

<time pubdate datetime="2010-11-15T00:00:00">foo</time>

Produces:

<blockquote>
Error: Bad value 2010-11-15T00:00:00 for attribute datetime on element time: The literal did not satisfy the date format.
</blockquote>

Using the non-shortened attribute pubdate="pubdate" makes no difference.