Your future and past in the blink of an eye
I'm night and I'm day, I'm land and I'm sky
The sun or the moon, neither or each
I'm easy to find yet harder to reach
I smother you softly from ever afar
I'm on no map, you can't get here by car
Travel by boat, by plane or by train
Any which way is always in vain
You are
the horizon
Your future and past in the blink of an eye
I'm night and I'm day, I'm land and I'm sky
The horizon is sometimes a metaphorical divider for past/future,
or where land and water or day and night meet (sunrise/sunset)
The sun or the moon, neither or each
Sunrise/set and moonrise/set happen at the horizon, so you might see either
or, while they're in the sky, you'll see neither there
I'm easy to find yet harder to reach
You can always see the horizon but you'll never reach it
I smother you softly from ever afar
Horizons are limits, always in the distance
I'm on no map, you can't get here by car
Travel by boat, by plane or by train
Any which way is always in vain
All of these are true of the physical horizon.
Are you:
Light
Your future and past in the blink of an eye
Light is how we see the universe, light which hits the eye. There is a 3 nanoseconds delay that occurs in your brain to process complex information from eye and other sources. Which means that when the light hit you and you process it, you are living 3 nanoseconds in past while in that time, your future already hit your eyes.
The sun or the moon, neither or each
Natural Light is from the Sun or the moon or neither of them.
I'm easy to find yet harder to reach
The speed of light is easy to find (You can do it in home with a microwave) but it's much harder to reach.
I smother you softly from ever afar
Light from every source in the universe is hitting us, they are just not bright enough for us to notice. In fact, the microwave radiation after the Big Bang is currently hitting us, none of us see it.
I'm on no map, you can't get here by car Travel by boat, by plane or by train Any which way is always in vain
Light is everywhere but on no map. Any attempt on reaching a photon, or reaching speed of light is in vain.
Are you...
A dream/dreaming/dreamland?
Because...
Dreams can be anything. All things great and small, good and bad -- whatever you can think, you can dream, and then some.
You can't get there because...
It is not a physical place you can go. You can find dreams anytime you sleep, but (most) people cannot actually control what happens in their dreams.
And the line...
"I smother you softly" may refer to the fact that dreams control you instead of the other way around. You cannot do anything else while you are asleep. It could also (albeit unintentionally?) refer to sleep apnea, a condition which actually does smother you in your sleep.
"...from ever afar" could also mean that the more time that has passed once you've awoken, the less you are able to remember about a dream, although it may frustrate you.
A simple answer:
Are you:
A star?
Your future and past in the blink of an eye
Light takes so long to reach the earth, that by time we can see the stars, they are already ancient (our past). They also provide insight into the future of our solar system
I'm night and I'm day, I'm land and I'm sky
Stars are in line of sight (yet not visible) during both day and night. They also exist below us (land) and above us (sky)
The sun or the moon, neither or each
While the sun and moon are not (historically) seen as stars, they are both light sources similar to the stars.
I'm easy to find yet harder to reach
Obviously, stars are easy to see, yet hard to reach
I smother you softly from ever afar
Although the stars are incredibly far away, they still shine a soft light
I'm on no map, you can't get here by car
There are no directions to the stars, and getting to the stars in a car would be somewhat difficult
Travel by boat, by plane or by train Any which way is always in vain
I think the logic here is pretty clear
Are you:
Attention / Focus
Your future and past in the blink of an eye
Each shift in attention represent one step forward in time
I'm night and I'm day, I'm land and I'm sky
The sun or the moon, neither or each
You can place your attention on anything, where ever it happens to be.
I'm easy to find yet harder to reach
Maintaining Attention (Focus) is notoriously difficult
I smother you softly from ever afar
Your attention must always be on something, even as you drift
I'm on no map, you can't get here by car
Travel by boat, by plane or by train
Any which way is always in vain
An attempt to redirect your attention to find the answer to this riddle while operating a car, boat, plane, or train would result in death (be for nothing).
