Which Pokémon games first introduced an internal clock with day-and-night gameplay?

Pokémon Gold and Silver first introduced an internal clock with day-and-night gameplay.

Released for the Game Boy Color in Japan in 1999, Gold and Silver added a time system that tracked the in-game day. Certain events, encounters, and gameplay conditions could vary according to the time of day, making the world feel more dynamic than in the first-generation games.

The games also introduced the Johto region, 100 new Pokémon, breeding, held items, Pokémon eggs, and a distinction between physical and special moves based on move type. Their clock was one of the most notable additions because it linked the game world to a persistent daily cycle.

Ruby and Sapphire and later games expanded the idea with their own time systems and mechanics. They were not the first Pokémon games to introduce day-and-night gameplay, however; that distinction belongs to Gold and Silver.