There's a note in the pages of the book. (джен)

First year of reign of Queen Rhaenyra Targaryen, First of Her Name, shown through her correspondence with her lord-husband, Ser Laenor Velaryon, and one notable, though not especially significant council meeting.
The fourth part of the series where Viserys died before Rhaenyra and Laenor left for Dragonstone, and so Rhaenyra was crowned much, much earlier.