What feature did Pokémon Gold and Silver introduce that changes with real-world time?

Pokémon Gold and Silver introduced a day-and-night system that changes with real-world time.

The feature divides the game world into time periods such as morning, day, evening, and night. The period shown depends on the clock in the Game Boy Color system, making the same location look and behave differently at different times.

Time affects wild Pokémon encounters, certain character events, and some evolutions. For example, several Pokémon evolve only when friendship is high and the game clock indicates daytime or nighttime. The system made the world feel more continuous than the first-generation games.

Gold and Silver were the second-generation Pokémon games. They also introduced the Johto region, 100 new Pokémon, Pokémon breeding, held items, and the Dark and Steel types. After completing the Johto story, players can return to the Kanto region from the original games.