Which Pokémon games first introduced a clock-based day-and-night system?

Pokémon Gold and Silver first introduced a clock-based day-and-night system.

The games used the Game Boy Color’s internal clock to track real-world time and the day of the week. Different times could affect which Pokémon appeared, how certain locations looked, and when events occurred. This made the game world feel more persistent than the fixed-time environments of the first generation.

The feature was tied to other second-generation additions, including weekday events, Pokémon that held items, new Poké Ball types, and the Johto region. Players could also travel back to the Kanto region after completing the main Johto adventure.

Pokémon Ruby and Sapphire retained a real-time clock for events such as tides and berry growth, but they did not distinguish between day and night in the same way. This is why Ruby and Sapphire are not the correct answer, despite continuing the clock-based design in a more limited form.