To display human readable dates on your blog, you have to use the human_time_diff() function. The following piece of code will show a post date like "Posted 6 days ago".
Paste it anywhere within the loop, save the file, and you're done.

Posted <?php echo human_time_diff(get_the_time('U'), current_time('timestamp')) . ' ago'; ?>

Credits: PHP Snippets.
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