1. Digimon Frontier: The Five Legendary Kids
In an episode of Digimon Adventures 01, it was noted that there had been Digi-Destined kids BEFORE Taichi and the rest. And there had been a shadow of five kids. Of course, looking closely, it's a shadowed sillhouette of Taichi and a few others, but that's a trivial detail. The point here is that there had been FIVE kids in the ancient legends.
Digimon Frontier had exactly five Digi-Destined kids to hold the ancient digimon spirits. Kouichi isn't a Digi-Destined technically because he had never been chosen by Ophanimon herself. Rather, he had been forced to hold one of the spirits, and he wasn't entirely alive either. At the end of the series, the digital universe was corrupted, but the digital codes restored it. Later, Bokomon said that he would write a story about them, and they would be remembered as legends forever.
The time spent in the Digital World had been quite long, but that equalled much less than a few hours in the real world. The kids had arrived in the Digital World when Kouichi fell down the stairs, and came back in the last episode when he was in the hospital. The time concept between the two worlds were different
2. Digimon Adventure 01: The Damaged Concept of Time
Yup - that's it. This may have taken place a few years after Digimon Frontier, but that's actually millions in the Digital World, which is more than enough time to regenerate and change. Because Apocalypsemon-thingy appeared and nearly destroyed everything, the time concept between the two worlds became the same - with the clock ticking synchronized.
2.5 Digimon Tamers: Ken
Some time BEFORE the end of Digimon Adventure 01, Ryou and Ken had been travelling in the Digital World. Why? Firstly, Ken snuck into his brother's room, had gotten his digivice, was transported into the Digital World, and came back around when his brother stormed on him for touching his things. There are also flashbacks of Ken travelling with Ryou, and he had been wearing the same clothes from when he was first transported. This means that the time concept had NOT been corrupted yet, and no matter how many days he spent in the Digital World, it was less than a few hours in the real world.
3. Digimon Adventures 02: Digimon Alongside Humans
We all know that
crappy ending of this series. Digimons and humans all become friends and yadda yadda yadda. However, I assume that there might have been trouble for Digimons to entirely fit in the real world, so there might have been a period where they were checking on the links BEFORE everyone grew up and Matt went to Mars with Gabumon.
3.5 Digimon Tamers: Ryou
Ryou was an experienced trainer, and probably Ken's mentor. They never confirmed him in the anime, though. After the concept of time was broken in Digimon Adventures 01 and the world saved, Ryou returned to the real world. Two to three years later, Digimon Adventures 02 took place, and digimon was well known throughout the nation. Of course, portals leading to the Digital World appeared here and there, and Ryou was aware of them. However, before digimons were introduced into the human world properly, humans wanted their children to be familiar with digimon and therefore invented the card games. Ryou being familiar with digimon already, had won card tournaments, and already had a strong bond with his Cyberdramon. He's also the one to guide the other kids in the Digital World.
Now after Yamaki and the others sealed the Digital Gate, Takato found out there was still a portal open. (hinted in the last episode). This means that the grown-ups didn't seal it off entirely, but wanted to leave possibility to work with Digimons. Years had gone, and now T.K. is finished with his novel on Digimon, and it's the era where Digimon and humans coexist.
4. Digimon Savers: Digimon Invasion
However, due to the massive amount of resourses and information data, Kurata had invaded the Digital World, and due to transignal problems, there had been issues with portal gates. Digimon were now being sent back to the world for people's safety, save for a few under the employment of DATS. The time concept is corrupted as it had been - Ikuto's age confirms that.