Hi, Adrian,

The fine manual is not yet up to date with the new exporter, so when you're
bitten, check http://orgmode.org/worg/org-8.0.html.

The "@" html-tag quoting has been replaced with a generalized "export
snippets" syntax (currently described at
http://orgmode.org/worg/org-8.0.html#sec-8-1).

This works:

#+MACRO: datetime @@html:<span class="datetime" title="$1">$2</span>@@