Which Pokémon games introduced the real-time clock and day-and-night system?

Pokémon Gold and Silver introduced the real-time clock and day-and-night system.

Released for the Game Boy Color, Pokémon Gold and Silver added an internal clock that tracked the time of day and the day of the week. This made the Pokémon world feel more dynamic than the settings in Pokémon Red and Blue, where encounters and events were not governed by a clock.

Time affected gameplay in several ways. Some Pokémon appeared only during particular periods, while certain events and characters followed schedules. The games also introduced a Pokégear that functioned as a watch, map, radio, and telephone.

Pokémon Ruby and Sapphire retained a real-time clock for features such as berry growth, but they did not distinguish between day and night. Pokémon Gold and Silver are also the first-generation-two games and introduced 100 new Pokémon, bringing the Pokédex total to 251.