> The best time to start terraforming a planet is 500 years ago.

For context, it took an estimated three-quarters of a billion years to oxygenate Earth's atmosphere. Even a speed-run of that is ... considerably longer than a few centuries.

source: https://en.wikipedia.org/wiki/Great_Oxidation_Event

(to be fair, Mars is quite rusty already, it has a head start on the Early Earth in that regard)