Which feature did Pokémon Gold and Silver introduce using an internal clock?

Pokémon Gold and Silver introduced a real-time day-and-night cycle using an internal clock.

The games track the current time and day of the week, and some events change according to that information. Certain Pokémon appearances, characters, and activities depend on whether it is morning, daytime, or night. This made the game world feel more persistent than the fixed-time environments of the first generation.

Gold and Silver were released in Japan on November 21, 1999, as the second generation of the main Pokémon series. They were designed for the Game Boy Color and added 100 new Pokémon, bringing the total Pokédex count to 251.

The same games also introduced Pokémon breeding, held items, the Steel and Dark types, and the Johto region. The clock system should not be confused with online connectivity: Gold and Silver still relied on local Game Link Cable features for trading and battling.