Judging by the rough timing of his first serialization, probably the 2000's in general which kind of tracks considering that decade Japan had a lot of crime. But they do give a rough location which is within walking distance of a train station at or near Haneda Airport
EDIT: Doing a bit more research, it's almost certainly somewhere in Minato. There was a lot of gang activity in Shinjuku, Shibuya and Minato in that time frame apparently