Answer:
What measures out time. Until in time all is smashed to it?
I am a box that holds keys without locks, yet they can unlock your soul.
This thing runs but cannot walk, sometimes sings but never talks. Lacks arms, has hands; lacks a head but has a face.
You use a knife to slice my head. And weep beside me when I am dead.
The root tops the trunk on this backward thing, that grows in the winter and dies in the spring.
What do you purposefully put lots of in and on your body, but run away from when you encounter it outside?