Which feature did Pokémon Gold and Silver introduce to the main series?

Pokémon Gold and Silver introduced a day-and-night system to the main Pokémon video game series. The in-game clock reflects real-world time and affects events, encounters, and certain evolutions.

The second-generation games were released in Japan in 1999 for the Game Boy Color. Their Johto setting added 100 new Pokémon, two new types—Dark and Steel—and mechanics such as breeding and held items.

Time-sensitive gameplay made the world feel more persistent. Some Pokémon appeared only during particular periods, while certain characters, events, and evolutions depended on whether it was morning, day, or night. The system also helped distinguish Gold and Silver from the first-generation games.

Later entries changed and expanded the clock mechanics, and some generations temporarily omitted or altered them. The day-and-night system should not be confused with Mega Evolution, which debuted much later in Pokémon X and Y, or Double Battles, introduced in Generation III.