<!-- schema.org markup for book in stock -->
<link itemprop="availability" href="http://schema.org/InStock">

<!-- schema.org markup for book out of stock -->
<link itemprop="availability" href="http://schema.org/OutOfStock">

<!-- here is how you mark a date to ensure it is interpreted unambiguously -->
<time datetime="2011-04-01">04/01/11</time>