It's the year 2009, for normal people anyway. For the DenLiner gang, it's what ever yea they want it to be; 2009, 1921, or even 2031, there's really no limit. Evil Imagin's are becoming less and less common, and Imagin's teaming up with people is becoming more and more common, of course, no one has as many as the origenal Den-O himself, Nogami Ryoutaro.
The in the year 2054, Ryoutaro's grandson, Nogami Kotaro is the New Den-O, although he spends more time fighting in the past than his present. But, he's not alone in being a singularity point in his time. He has a good friend, Kagami Remi, who is also. Remi though, well, she doesn't fight. Often. She does sometimes use Cianna, her own Imagin, as a bow to distract the Imagin to give her friend and his grandfather help.
No matter what has happened, time needs to be protected, from whatever, and this is what the Riders do in this world. They fight to keep the time right, mostly. And to keep Imagin from trying to kill people and such.