You have to change the format around line 89 of your /skins/yourskin/_item_comment.inc.php:

<?php $Comment->date() ?> @ <?php $Comment->time( 'H:i' ) ?>

There you change the date and time format for comments. I'm assuming you're using 2.x versions, although you don't say.

Yes you are correct about the version and your reply works. Just changed it, thanks a lot. One more thing please, how about the date on the posting, just below the title? Where can I change the format?

I found the format of the time into posts.main.php. Thanks,it works.