Which Pokémon game generation introduced the day-and-night cycle?

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

The feature debuted in Pokémon Gold and Silver for the Game Boy Color. The games tracked the real-world time of day and changed the world accordingly, including different wild Pokémon encounters, character schedules, and events that depended on morning, day, or night.

Generation II also expanded the original formula with Pokémon breeding, held items, Pokémon genders, a new region called Johto, and 100 new Pokémon. The time system connected several of these additions to exploration and evolution: Espeon and Umbreon, for example, evolve from Eevee through friendship at different times of day.

The cycle is sometimes associated with Generation I because the first games established the series’ exploration structure. However, Red, Green, Blue, and Yellow do not contain a functioning real-time day-and-night system. Gold and Silver were the games that introduced it as a major gameplay feature.