Light years is a distance, why not light 500 seconds is a distance. Physicists have confused the terminology. I read there are "light miles" meaning the time it takes light to travel a mile. We use a distance measure to denote time and time measure to denote distance.
Let me bring up another point that is confusing folks. Displacement denotes a vector quantity, where "direction" is involved. Direction is the distinction between vectors and reals quantities. Another confusion is the velocity of light as opposed to the speed of light. This distinction could explain the M&M experiment. Light is a real number and ct is a real number, not a vector or displacement. Now real numbers don't have directions like vectors so light has the same "speed" in all directions. I wouldn't argue the point but I was not surprised that the M&M Experiment "failed", M&M looked for vector compounding from a real number and didn't find it. Real numbers don't compound like vectors. Thus light is constant in all directions and the M&M Experiment failed. To this day physicists are talking about the velocity of light as if light was a vector velocity. Light is a real number time is a real number ct is a real number, a distance not a displacement. reals and vectors are different. Like in complex numbers reals are different from imaginaries.