Answer to a Riddle?

[Bilbo Baggins]

A strange man I met asked me this riddle:

This thing all things devours;
Birds, beasts, trees, flowers;
Gnaws iron, bites steel;
Grinds hard stones to meal;
Slays king, ruins town,
And beats mountain down.

I don't have enough time to figure it out for myself; anyone have an answer?