Death Note: Desu nôto2006

Ryuk:
The second difference between us will probably be of even greater interest to you, though this has less to do with lengthening life and more to do with cutting it short.

Light Yagami:
Whadda mean "cutting it short?"

Ryuk:
Shinigami can tell what a person's name is by just looking at their face. We know instantly. And do you have any idea why? Because, through a shinigami's eyes, a human's name can be seen above their head. Not only that, we see their lifespan as well.

Light Yagami:
Their name and lifespan?

Ryuk:
That's right. I'm able to see your name and lifespan right now. If I converted that to human time, I could tell you how many years you got left - but there's no way I'd ever tell you that.