A web page could be encoded in nothing but div and span tags. However, search engines do not process those types of pages very well. They like material to be marked with tags that identify the type of material e.g. Headlines, Paragraphs …
From a page design standpoint a time tag is the same as a span tag. But looking outside of the page itself it provides information to the outside world about that particular piece of data, page or article.
The simplest format is <time datetime=”2009-10-22″ pubdate>October 22, 2009</time> or <time datetime=”2009-10-22″ ></time>. An RSS type of format may also be used <time datetime=”2009-10-22T13:59:47-04:00″ pubdate> October 22, 2009 1:59pm EDT </time>
This information is very useful to researchers and greatly expands how search engines can offer up search results. Timeline searches are not exactly mainstream right now but that does not mean they should be ignored.