X-Plane 12 uses an IGRF13-based grid for magnetic variation between July 2021 and June 2024. Within these dates, X-Plane’s magnetic variation will change over time based on real time.
Magnetic variation in X-Plane depends on the UTC date of the operating system’s clock, not the simulated time in X-Plane. That means, you can chose the date and time of your flight in X-Plane to be whatever you like to fly over your favourite seasonal forests, but the magnetic variation will always reflect the current real time.
Note that if your operating system’s clock is set earlier than July 2021 for any reason, the annual change in magnetic variation is capped there. It is generally required that you keep your operating system clock in sync with real time, especially if you want to use up-to-date navdata.