Heading tags (H1 through to H6) are given more weight by search engines than regular body copy. So they should be used wisely to reinforce the page’s overall keyword theme. The posting date should never be within an H1 tag, because you’re NOT trying to rank well in the engines for a date. Instead, wrap an H1 tag around your category name or tag name on your category page or tag page, respectively. Then make the titles of your blog posts H2 tags, so that on a category or tag page, the category/tag words can be the only H1 on the page and thus can convey greater emphasis than the post titles.
You could even take this idea a step further and make the post title display within an H1 tag if it’s a Sticky post, and within an H2 if a normal post. (See my last post for more on the benefits of Sticky posts for SEO.) Check out this example category page on BusinessBlogConsulting.com where I did just that.