In Blogger.com many conditional value used for extracting data from blog in a specific meta attributes. Such as
<meta name="description" expr:content="metaDescription"/>
here metaDescription retrieve the page description from the blog.
What is the conditional value for replacing date published in meta content attributes? Like
<meta name="date_published" content"??????"/>
To retrieve published date of posts and use that as meta tag you have to rely on Schema structured data markup.
To do that copy and paste this following codes just below post hentry uncustomized-post-template div elements.
<meta expr:content='data:post.dateHeader' itemprop='datePublished'/>
You can also add this following codes to tell the search engines when did you last update your blog post.
<meta expr:content='data:post.lastUpdatedISO8601' itemprop='dateModified'/>
To check if this retrieving the post publish date and post update date properly, simply go to Google Structured Data Testing Tool and fetch the url of your blog post.
Here @Unor described in details why you should not put this schema meta tags inside <head> tag.
I think date data only available on post widget like blog widget and popular post widget. It's not possible in head tag.
