It’s better to create a child theme first :Codex:Child Themes
Once that is done your style.css should look something like :
/*
Theme Name: Twenty Eleven Child 1
Theme URI: http://example.com/
Description: Child theme for the Twenty Eleven theme
Author: Your name here
Author URI: http://example.com/about/
Template: twentyeleven
Version: 0.1.0
*/
@import url("../twentyeleven/style.css");
I’m not sure this is the best solution, but it works.
Then create a functions.php in your child. Copy lines 580 to 592 form the original twentyeleven functions.php to your new functions.php.
Change
printf( __( '<span class="sep">Posted on </span><a href="%1$s" title="%2$s" rel="bookmark"><time class="entry-date" datetime="%3$s" pubdate>%4$s</time></a><span class="by-author"> <span class="sep"> by </span> <span class="author vcard"><a class="url fn n" href="%5$s" title="%6$s" rel="author">%7$s</a></span></span>', 'twentyeleven' ),
to
printf( __( '<span class="sep">Posted on </span><a href="%1$s" title="%2$s" rel="bookmark"><time class="entry-date" datetime="%3$s" pubdate>%4$s</time></a>', 'twentyeleven' ),
