But! I dont like doing that with the only way I have found as described below. (I guess you were expecting that word to be written first.)

Last updated/modified date of blog post isnt available for normal bloggers on Blogger or template developers to perfect their amazing templates. If you follow the no modification rule for blogging, then you wont need it since you wont press Publish button for second time for a post. If you are a blogger like me, you want to fix your posts, even a typo, you may want this kind of date to be shown to your readers, especially you make a major change to your post and you do not want to add a section for recording changes you have made to the post.

Lets stop the wordy description for a second and see the action part, shall we?

Here is a popular post of mine and I have made few changes a few times,

Dates will be shown here.

The basic code using jQuery is:

var blogID = '3803541356848955053';
var postID = '1127403767086065';
$.getJSON('http://www.blogger.com/feeds/' + blogID + '/' + 'posts/summary/' + postID + '?alt=json-in-script&callback=?', function(data) {
    // doing blah blah blah here

It utilizes Blogger API to retrieve posts summary data in JSON format. Its easy to parse the date string and to re-format it to be a more human-readable format any style you want. With this method of showing dates, it requires two parameters, blog ID and post ID. They are easy to find in DOM. If its a standard Blogger template, they are in one of <meta/> if its a single post page. For home or archive pages, etc., you can find them in if Email Post feature is enabled. Or you can modify template to include a numeric post ID using Layouts Data Tags.

Is this the only way? I am afraid so. I cant find any reference for last updated in data tags, or any useful posts on help forums. I didnt ask on forums, because I dont expect a Blogger staff will reply to tell me if there is a such data tag or there is not. I also tried to make up few data tags to see if I could find a secret tag for modified dates, but I found none.

Give me an hour, I could have a working script to be used on my blog, but I wont do it. Because multiple posts require multiple Blogger API requests, and I doubt many people will be really interested in knowing when I update my posts. If Blogger API can allow selecting multi-posts using post IDs as if how Disqus shows the number of comments for each posts, I might write the script.

I also believe in showing to users how long have a post been published and last updated is important, because a post about computer or technology, it could be outdated very quickly. Letting readers be aware of the post age might be helpful, because they might leave a comment saying You stuff isnt working, which most likely implying your stuff is a crap. I had wanted to implement a reporting system using Google Analytics event feature, or using Google App Engine, but I never did.

Anyway, if you are so wanting to have Last Updated on your blog and your blogs template is close to standard/official Blogger template, leave a comment (DO NOT leave your blog link in comment body). If many people also join you, I will leave a comment so you will receive an update email, which to let people know I am going to write a script for your guys and you can email me your blog link.