time

I can run swiftly and silently when you want me to stay still,
I can move slowly and cautiously and am yours to fill.
you look at me often and yet you always forget me,
I am the most feared killer, yet you can't live without me.
sometimes you have me for all to spare,
yet when you need me, I am not there.
you can waste me, or cherish me, you choose the track,
but once you're done you can never get me back.