Answer:
I am in the past, never in the future. I don't exist, but have existed. I saw what you saw, and this is what I will ever see. What am I?
What has one eye but cannot see?
I am round. I have only one line. Circle is not my name indeed. What am I?
I am the place to turn when nature calls.
What is black when you buy it, red when you use it, and gray when you throw it away?
My days are numbered.