Is there a standard for tagging a html document with create date and modified date?
Ketil
If you're willing to use Schema.org microdata, you could use dateCreated and dateModified.
Something like this:
<body itemscope itemtype="http://schema.org/WebPage">
<p>
Created on <time itemprop="dateCreated" datetime="2014-12-29T04:19:22">December 29th, 2014</time>
</p>
</body>