I think this needs a plugin, one that correctly hooks to the postbit.
Actually, you could try it like this:
Postbit Classic
{$ignore_bit}
<a name="pid{$post['pid']}" id="pid{$post['pid']}"></a>
<div class="post classic post{$usergroup['gid']} {$unapproved_shade}" style="{$post_visibility}" id="post_{$post['pid']}">
<div class="post_author scaleimages">
{$post['useravatar']}
<div class="author_information">
<strong><span class="largetext">{$post['profilelink']}</span></strong> {$post['onlinestatus']}<br />
<span class="smalltext">
{$post['usertitle']}<br />
{$post['userstars']}
{$post['groupimage']}
<script>
var timestamp = {$post['lastvisit']}; // replace your timestamp
var date = new Date(timestamp * 1000);
var formattedDate = ('0' + date.getDate()).slice(-2) + '/' + ('0' + (date.getMonth() + 1)).slice(-2) + '/' + date.getFullYear() + ' ' + ('0' + date.getHours()).slice(-2) + ':' + ('0' + date.getMinutes()).slice(-2);
</script>
</span>
</div>
<div class="author_statistics">
{$post['user_details']}<br/><script>document.write(formattedDate);</script>
</div>
</div>
<div class="post_content">
<div class="post_head">
{$post['posturl']}
{$post['icon']}
<span class="post_date">{$post['postdate']} <span class="post_edit" id="edited_by_{$post['pid']}">{$post['editedmsg']}</span></span>
{$post['subject_extra']}
</div>
<div class="post_body scaleimages" id="pid_{$post['pid']}">
{$post['message']}
</div>
{$post['attachments']}
{$post['signature']}
<div class="post_meta" id="post_meta_{$post['pid']}">
{$post['iplogged']}
</div>
</div>
<div class="post_controls">
<div class="postbit_buttons author_buttons float_left">
{$post['button_email']}{$post['button_pm']}{$post['button_www']}{$post['button_find']}{$post['button_rep']}
</div>
<div class="postbit_buttons post_management_buttons float_right">
{$post['button_edit']}{$post['button_quickdelete']}{$post['button_quickrestore']}{$post['button_quote_fast']}{$post['button_quote']}{$post['button_multiquote']}{$post['button_report']}{$post['button_warn']}{$post['button_purgespammer']}{$post['button_reply_pm']}{$post['button_replyall_pm']}{$post['button_forward_pm']}{$post['button_delete_pm']}
</div>
</div>
</div>