Which Pokémon generation introduced the day-and-night system?

Generation II introduced the day-and-night system in the Pokémon video games.

The feature debuted with Pokémon Gold and Silver, released in Japan in 1999 for the Game Boy Color. The games tracked time in relation to the real world, allowing events and encounters to vary between morning, daytime, evening, and night.

Generation II also expanded the series with the Johto region, 100 new Pokémon, breeding, held items, and the Dark and Steel types. Its time system made the world feel more persistent than the original games, where gameplay took place without a changing clock.

The feature is sometimes incorrectly credited to Pokémon Crystal or a later generation because Crystal added other notable enhancements. Crystal was an expanded version of Gold and Silver, not the original source of the day-and-night mechanic. Later games continued to use time-based events, although the exact implementation changed between generations.