Which Pokémon game introduced the Johto region and the day-and-night cycle?

Pokémon Gold and Silver introduced the Johto region and the day-and-night cycle. The second-generation games used the Game Boy Color's internal clock to make time affect encounters, events, and some Pokémon evolutions.

The cycle was more than a visual change. Certain Pokémon appeared at particular times, some characters followed schedules, and a few story events depended on whether it was morning, day, or night. This made the game world feel more persistent than the original Red and Blue adventures.

Gold and Silver also introduced 100 new Pokémon, breeding, held items, the Dark and Steel types, and a connection to the Kanto region. These features made the games a major expansion rather than a simple sequel with a new map.

Pokémon Ruby and Sapphire later changed how time was handled, and the day-night system returned in later generations. The feature is therefore most strongly associated with Gold and Silver as its original main-series introduction.