The first stars formed when distances were 9% of their present size. The solar system began forming when distances were 70% of what they are now. Andromeda galaxy will make a first pass through Milkyway when they're 129% of present. Which do you find more meaningful and intuitive for tracking the evolution of the universe, percentages like those or numbers of years? Universe history (say from recombination onwards) is often laid out using years to parametrize event, but in some of Lineweaver's diagrams the scale factor (denoted "a") is used as well. If you have any thoughts about this: reasons to prefer one or the other, I'd be interested to learn what you think--and which parametrization is more intuitive for you. If you have some other parameter that works better please post it, and discuss. We are not talking about the first few milliseconds of expansion, or the first three minutes, here, but about history after recombination and the emission of the ancient CMB light.