Consensus among cosmologists is that the universe is 14 billion years old. It doesn't help much in our reckoning of time to know a life span is essentially nothing on a cosmic scale.
Clocks are mechanical contraptions calibrated to predicable phenomena like the rotation of the Earth which conveniently stages a predicable sunrise and a sunset.
It doesn't help our understanding of time that the rotation of the Earth is slowing imperceptibly due to the tidal friction of its Moon. Fortunately for our comforting sense of clockwork consistency, the slowing rotation of the Earth is negligible. Today's days are only about 1.7 milliseconds longer than 100 years ago.
Time is a common variable that pops up in classical mechanics and the mathematics of physics. Still, physicists seem far from understanding, let alone explaining, the flow of time.
Time exists in physics, but the flow of time does not. Physicists do not understand the flow of time. In any given coordinate system, we can be at rest in space, but in that same coordinate system, we cannot be at rest in time. Time has this qualitatively different feature. It progresses.A moment is central to our sense of reality. Yet a moment is ephemeral in the most infinitesimal sense of the word.
— Richard Muller
Einstein despaired of his inability to explain the flow of time. But Einstein, despite his despair, moved forward and showed that the rate of the flow of time depends on both velocity and gravity. That suggests strongly that the flow of time does not originate in the human mind, but has a true external physical reality.A reassuring, yet possibly erroneous analogy of a moment in time is to visualize the still frame of a movie sandwiched with an imperceptibly small delta before the next the still frame.
— Richard Muller
Every moment is as real as every other. Every 'now,' when you say, 'This is the real moment,' is as real as every other 'now' - and therefore all the moments are just out there. Just as every location in space is out there, I think every moment in time is out there, too.For now it must suffice to consider time a sensory snapshot meant to be experienced to the fullest extent possible.
— Brian Greene
Who would then deny that when I am sipping tea in my tearoom I am swallowing the whole universe with it and that this very moment of my lifting the bowl to my lips is eternity itself transcending time and space?
― D.T. Suzuki