Which Pokémon game introduced the series’ day-and-night system?

Pokémon Gold and Silver introduced the Pokémon series’ day-and-night system.

The games use a real-time internal clock that tracks the day and week. The time can affect which Pokémon appear, when certain events occur, and how parts of the world look. This made the game world feel active even when the player was not following the main story.

The feature was part of Gold and Silver’s broader expansion of the original formula. The games also added Pokémon breeding, held items, berries, and the Dark and Steel types. Their Johto setting was designed around a connected world that changed over time.

The clock system should not be confused with the Nintendo DS titles’ later graphical improvements or with Pokémon Crystal’s additional features. Crystal expanded the second-generation formula, but Gold and Silver introduced the core day-and-night mechanic.