Which game pair introduced real-time day-and-night mechanics to the main Pokémon series?

Pokémon Gold and Silver introduced real-time day-and-night mechanics to the main Pokémon series.

Their internal clock tracks the time and day of the week, allowing gameplay conditions and certain events to change depending on when players play. Some Pokémon appearances and encounters are therefore connected to morning, daytime, or nighttime.

Gold and Silver were released in Japan in 1999 as the second generation of the core series. They are set mainly in the Johto region, but they also allow players to return to Kanto after completing the Johto storyline.

The games also introduced Pokémon breeding, held items, shiny Pokémon, the Steel and Dark types, and a split between Special Attack and Special Defense. Later games retained and expanded the clock system, so it can be easy to remember the feature without remembering that Gold and Silver introduced it.