Sometime after the 12th/13th cycle…

Y’Shtola is a seeker of the Sun Miqo’te conjurer. She aids the Warriors of Light in the resistance against the Garlean Empire.

When did Materia call Y’Shtola to World B? Most likely during the time immediately after World Destruction. Before transporting to the new Eorzea, she probably finds herself in World B.