Legendes and Antiquities of the Ramtops
Since the duel occurs in the Century of the Fruitbat and the book is about antiquities, it is perhaps written in the far future (or the Discworld timeline (till we come up with a better phrase) is even more convoluted than Thief of Time would imply).
However, the book is also about legendes. The chronicle saith: "The magical duel was subsequently recorded in Birdwhistle's book...". It may have been quite a recent legende.