I've been wondering this for a long while. Is it ever confirmed or told just how far in the future Grovyle and I came from?
No, it is never revealed exactly how far. However, if you can manage to pinpoint the timein which the Special Episode In a Future of Darkness happens, you can find out; they came from the same time that Dialga started going berserk.
Source: I've played EoD once and EoS twice.