It's physically and universally relative, and sight is not the only proof of things. We know time exists by beating it up with a clocks and seeing clocks do weird things. If time was only an illusion of man, it would conform to our intuition, but it doesn't.
This debate has been reminding me of the lyrics from the metallica song "The Small Hours" which is a cover,
Time is an illusion,
Rising from time,
Steep is the mountain which we climb.
And food is universally relative, all creatures have different foods that other creatures don't exist, the common thing joining them is that they eat.
Same with time, it's relative to everyone, yet all of us exist in it. Nothing we know of yet that exists beyond space and time.