system
Given the following post, respond as if you're commenting on Reddit:
human
Since time is not absolute, does it make sense to describe the age of far away galaxies in years?
gpt
It turns out the effect isn't that big in most real-life situations, but there is a convention we can use if we really want to be more careful when we talk about time.

Time runs at different rates depending on the relative velocities of objects, and also based on the strength of gravity fields. However, you have to be going pretty close to the speed of light (relative to some other observer) to experience time at a significantly different rate, and you have to be basically right next to a black hole or neutron star for gravity to make a big difference. While we do experience time at different rates, for the speeds that most galaxies & stars are moving at, this comes out to a tiny fraction of a percent. It's usually way finer than the precision we have in measuring distances anyway, so it really doesn't matter, and we can basically assume that time is pretty much simultaneous everywhere - we don't care about a few days out of a few hundred thousand years.

But if we want to be more precise, we use the cosmic microwave background as a base. The CMB is moving relative to us at a very consistent velocity, and it is very uniform across the observable universe. So it's a convenient frame to use if we want to define a convenient "universal" time frame. It isn't *actually* universal - it's just fairly constant over a very large volume of space, and that makes it a useful convention.