While playing around with my Google+ profile I noticed a posting about Google Authorship, i.e. linking postings from various web sites to your profile. Search results show your profile photo, if activated. This was nothing new for me. But nonetheless I tested the mentioned Google Rich Snippets Testing Tool with my web site and it threw the following error message multiple times:
Rich Snippets Error Missing required field "updated"
I quickly found out that the culprit was my theme “twenty twelve”. Its predecessor “twenty eleven” also led to this error messages, but other themes didn’t. So I dug myself into the PHP code (Appearance -> Editor) and narrowed it down to a single class definition in functions.php (ca. line 326) that had to be changed from “entry-date” to “updated”:
$date = sprintf( '<a href="%1$s" title="%2$s" rel="bookmark"><time class="updated" datetime="%3$s" pubdate>%4$s</time></a>',
I have not checked what the difference between “entry-date” and “updated” is, but I have not yet observed any negative consequences; so I will stick to the change. The code in the “twenty eleven” theme looks similar, so if you want to adapt it search for “entry-date” (only one occurrence) in the functions.php file.