Why was it 1929 when the last hero of GOSICK24 returned to Japan?

The novel is also 1929. The reason why the man came back is that the war is over, so he can only be a parallel world similar to reality.

Finally, the hostess said that a new world was born. I don't know if it can be understood that the normal development is the end of the 1945 war and the separation of men and women in this storm that swept the world (the prediction of Big Wolf).

But in the novel world, men and women fight to change the future, resulting in a new world (new future). Four years later, the war ended and men and women met.