new timestamp for every post?

hey guys,
i’m confused!

function test($content) {

	$timestamp = time();
	echo $timestamp;
	return $content;
}

add_filter('the_content', 'test');

I thought this will echo a new $timestamp for every post on my frontpage. However every post has the same timestamp.

What am i misunderstanding here? Is there a chance to get what i want and have a new timestamp ID for every post?

thank you.

edit: or is it simply possible to echo out each post’s ID with my plugin. That would be unique enough. I just need a printed unique identifier for every post.