system
Given the following post, respond as if you're commenting on Reddit:
human
When we talk about the age of universe or stars, in what reference frame are we measuring the time?
gpt
We use a frame of reference where the cosmic microwave background has no velocity, which is about 600 km/s from the velocity of the Sun. This is the "CMB rest frame".

In practice, most galaxies, stars, and planets move at <1000 km/s relative to each other, so relativistic effects are so small that they're completely buried in all the other sources of uncertainty. So it really doesn't matter much, but we use the CMB rest frame for consistency.

The CMB rest frame doesn't really have a huge amount of significance. All the matter in our observable universe came from a very small region in the early universe, so small that there was not really any change in velocity from one side of the region to the other. The actual value of that velocity doesn't mean anything - it had to be *something*, but that's it. If you were able to go well beyond our observable universe, you would see this value start to slowly drift away from the local value.