"mark" new posts as "NEW"?

you could add a small function to functions.php of your theme;
example:

function newly_posted() {
global $post;
$now = date('U'); $published = get_the_time('U');
$new = false;
if( $now-$published  <= 3*24*60*60 ) $new = true;
return $new;
}

and in your template, use:

<?php if( newly_posted() ) :
/*output whatever */
endif; ?>

http://php.net/manual/en/function.date.php