I don't recall hearing/seeing that the machine is limited in that way, just that that's the only increment of time by which MC has traveled. So far.
I don't think she looks 20 years older either, but I do think she looks notably older (not to mention harder, for lack of a better word--she's the one who killed the bad guy without hesitation) than 2019 Darci. And, of course, 2019 Darci is right there with you when you meet her, so unless someone has a Doppelmorpher (oops, wrong game), she has to be a time traveler. So from when? If it isn't from 2039, we're back to people traveling by something other than exactly 20 years at a time.
So back to Qwerty's question, if time travel isn't going away (and I doubt it is, unless the story ends unexpectedly early), and he doesn't go back to 1999, where/when does he go? Well, if we're sticking with the same basic cast, it can't really be before 1999 (both because most of them would be under 18, and because they would already know him when they meet him in 1999--but we've already seen that they don't). It probably isn't going to be after 2019, at least not by much--I just don't see much attraction in seeing the same girls, just older, unless you're going to get with their daughters. I'm making the assumption that it's the same basic cast, because it's already pretty big, and I really don't see Jestur wanting to create even more characters. So my guess would be to, or around, 2009, probably shortly before the accident. He probably doesn't see Scarlett (her nostalgia is of 1999, not of 2009) or Luna. He might see Khloe, maybe Valerie (though the closer to 2019 he messes around with Val, the harder it is to believe that she doesn't recognize him in 2019). Probably Jenny (I don't think that's just wishful thinking, though I'd sure like to see her again--and I don't think it's an undue amount of projection to think that MC has the strongest feelings for her among the 1999 LIs). But maybe he gets to know April?
