We die and we know it beforehand. We have a birth and we have a death, beginning to end: time is a line. We have patterns that change, the sun, the moon, the seasons, plants grow and then die and come back again: time is a circle. You. Look at yourself. Reading this, thinking stuff, remembering things, connecting thoughts, noticing surroundings, all in the changing now: time is phenomenal flux. You know this already like muscle memory because you experience it with your body: time is sensory perception. Death is a passage to another existence similar to this one except it goes on forever: time is endless duration. Something or someone or several someones made us and set us into a finite time line, but their world is eternal and non-linear: time is the distinction between creator and creature. There is no creator, eternality resides inside us, was never born and will never die: time is eternity of being.
How do we know this? We need to know death to know most of it. What is death? Only two things: a mystery and a certainty.