Given an estimate of the mass and mass distribution of the Milky Way we can compute the time dilation factor we expect assuming that the intergalactic space bey

Given an estimate of the mass and mass distribution of the Milky Way we can compute the time dilation factor we expect assuming that the intergalactic space beyond the galaxy has zero time dilation. However, I think we cannot easily measure or infer the absolute time dilation we experience due to the rest of the universe. But relative time dilation factors are enough for the sort of analysis that is done in timescapes.