Given that the girl who lost her digimon partner ends up being the adult woman who's working for the Government or whatever, I think it's safe to assume it happened anywhere from 5~10 years before season 1. I dunno how the timeline all adds up though, with the Dark Masters being involved. They were defeated/sealed away by the other kids with the girl, re-appeared after the season 1 kids beat Myotismon in the real world (which presumably resulted in many years passing in the digital world), and then are now reappearing after the reboot.
I guess the theory of digimon being reborn as eggs when defeated does kind of make sense. The Dark Masters were defeated, took a long ass time to mature into their mega forms, and then were defeated again before the reboot.