Well it's simple. I've seen it before on other forums, but I'm not sure if there is a plugin for it here.

 

Basically it records the time that user has spent online in days/hr/seconds, and then shows it under the username. Also it allows you to have it as a block.

 

If nobody has made this, would anyone be interested in making this for me? 

 

http://imgur.com/lnSjJvC

There was one: 

It was cut though because for IPS4 it never worked properly.