What timekeeping feature did Pokémon Gold and Silver add to the series?

Pokémon Gold and Silver added a real-time clock to the Pokémon game series.

The games use an internal clock to track the time of day and the day of the week. This system affects gameplay: certain Pokémon appear at particular times, some events occur on specific days, and several evolutions or encounters depend on when the player is playing.

Gold and Silver were released for the Game Boy Color in Japan in 1999 and internationally in 2000. Their time system made the world feel more persistent than the original games, where the game world did not continuously mirror the player’s real-world schedule.

The feature is sometimes described as a day-and-night cycle, but the real-time clock is the underlying system. It also supported other additions, including held items, breeding, berries, and 100 new Pokémon. Later Pokémon games changed or expanded the clock mechanics, but Gold and Silver established real-world time as a major part of the series’ design.