A light year is the distance it takes light to travel one year.
Light being a constant in a vacum travels 299792458 metres per second or 186 000 miles per second gives us a bunch of info.
"IF" it was possible to get a space ship to go the speed of light which it really doesnt seem to possible but lets suspend that and say IF than we can calculate the speed of the object at 186 000 miles per second.
From the outside perspective watching the ship.
Inside the ship is a different story.
Time changes the faster you go and approaching the speed of light would make it basically stop for the person inside the ship compared to the outside universe.
The I think was trying to point out that time dillation and its relativity to the planets, stars and universe around it.
When the space ship left its home 3 million light years away you can do the math based on the speed of light to determine how long it would take to reach us IF it was going the speed of light.
The occupants inside would be almost the same age when they reached us as when they left but the universe would have aged around them a lot.
What we dont know that kind of matters for this whole line of thought is.. did E.T go the speed of light to get here?
Perhaps they went sub light and its a generational ship.
Perhaps they warped space around the ship so space itself goes faster than light which physics allow for and the ship was carried to us faster than light.
IF you knew the speed of the ship when it left and when it arrived you could calulate how long ago in TIME it was when they left and when they got here.
You could also know how much the beings inside the ship aged.
A lightyear is the TIME it takes for light to go the DISTANCE of about 6 trillion miles going at the SPEED of 186 000 miles per second.
SteelyAlpha you could not be more wrong in the way you view a light year.
In fact distance, time and speed are all needed and define the term light year.