What real-time gameplay system was introduced in Pokémon Gold and Silver?

Pokémon Gold and Silver introduced a day-and-night cycle linked to real-world time.

The Game Boy Color games used an internal clock to track the time of day and the day of the week. This affected parts of gameplay, including which Pokémon appeared, when certain events occurred, and how the world looked during morning, day, and night.

The time system made Johto feel more dynamic than the first-generation games. It also supported events and encounters tied to particular periods, encouraging players to revisit places at different times. The feature became an important part of later main-series Pokémon games, although later systems handled clocks and calendars differently.

Weather effects are another Pokémon gameplay feature, but they were not the answer here. Friendship was also developed across the series, while Mega Evolution debuted much later in Pokémon X and Y. Gold and Silver are specifically associated with the introduction of a clock-based day-and-night system.