Currently listening to the biography on Musk, he jokingly accepts that his timelines are all off, but he doesn't feel too bad about it because he eventually hits the goals. When asked about comments that a timeline for an electric car took a couple years longer than he had predicted (like a 1 year prediction, and it took 3.5 years, or something like that) he said roughly "Oh yeah, that's crazy. But I didn't know that yet, I'd never done this. We got it done eventually, but that's an unrealistic timeline".

He says he makes timelines based off what he knows, and as he gets more data, he gets more realistic with the timelines. I agree he is getting better with the timelines. And he generally builds everything he talks about eventually.