What feature in Pokémon Gold and Silver used 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 time of day and day of the week while the player explored Johto. Some Pokémon appearances and in-game events changed according to that schedule, making the game world feel different in the morning, afternoon, and night.

This system was a major addition to the second generation because the original Game Boy games did not model a continuously changing time of day. Gold and Silver were designed for the Game Boy Color but remained compatible with the original Game Boy.

The feature is sometimes described simply as a clock, but its gameplay importance came from linking time to encounters and events. Later Pokémon games expanded the idea with weather, seasons, and more complex environmental systems, which are separate features from the original day-and-night cycle.