What major gameplay feature did Pokémon Gold and Silver introduce using a real-time internal clock?

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

The clock tracked the current time and day of the week, allowing parts of the game world to change according to when the player was playing. Certain Pokémon appearances and in-game events were tied to particular periods, making the world feel less static than in the first-generation games.

Gold and Silver also added Pokémon breeding, held items, berries, new Poké Balls, and a split of the original Special stat into Special Attack and Special Defense. These additions made the second generation more than a simple collection of new creatures.

The feature is sometimes confused with the later weather mechanics associated with Ruby and Sapphire. Gold and Silver’s defining time-based system was the clock-driven day-and-night cycle, while Ruby and Sapphire retained real-life time for events such as tides and berry growth but did not distinguish day from night.