Is Japan still running those Breeder reactors. If so, they have plenty of plutonium to make bombs. Nukes in 2 years, at the outside, from the day of a decision.
I strongly suspect that it's nukes in about 2 days from decision.
I would be shocked if Japan does not have all the elements of nukes ready to go in separate parts ready for assembly and, amazingly enough, the nukes just happen to fit on ballistic missles Japan has in the closet that just happen to have enough range to hold all of China and NK at risk.
Any takers on that bet :)