Which Pokémon games first added a real-time clock and day-of-week system?

Pokémon Gold and Silver first added a real-time clock and day-of-week system.

The games used the Game Boy Color's internal clock to track time. Morning, day, and night affected encounters and events, while particular days could trigger recurring activities or appearances.

This system made the game world feel more persistent than the fixed-time settings of the original games. It also helped distinguish Pokémon that appeared at different times and introduced planning beyond ordinary route exploration.

Pokémon Crystal retained and expanded the same time-based foundation. The clock is not the same as the anime's broad day-night storytelling, and it should not be confused with the battle feature introduced in later games.