Modify submitted by text in two different ways. The better way is in the preprocess function below.

node.tpl.php

  <span class="submitted"><?php print $submitted?></span>

   <?php if ($submitted) { ?>
    <span class="submitted">
        <?php
            print 'by ' . theme('username', $node) . ' | ' . format_date($node->created, 'custom', 'd/m/Y');
            if ($terms) {
                print ' | in ';
            }
        ?>
    </span>
    <?php } ?>

template.php

<?php
//template.php ways
function THEME_preprocess_node(&$vars) {

  $vars['submitted'] =  t('Posted by !username on !datetime',
        array(
        '!username' => $vars['name'],
        '!datetime' =>  format_date($vars['created'], 'custom', 'd/m/y'),
        ));
        
}
?>