Which Pokémon game first introduced a real-time day-and-night clock?

Pokémon Gold and Silver first introduced a real-time day-and-night clock to the main Pokémon games.

The Game Boy Color pair used an internal clock to track the time of day and day of the week. This affected gameplay: some Pokémon appeared at particular times, certain events changed depending on the day, and the game world visually shifted between morning, daytime, evening, and night.

The clock made the adventure feel connected to the player’s real-world schedule. It also supported new mechanics such as time-dependent evolutions and encounters, adding variety to exploration beyond the fixed world of the first-generation games.

Pokémon Crystal also uses the same time system, but it was released after Gold and Silver as their enhanced edition. Ruby and Sapphire retained a real-time clock for events such as tides and berry growth, yet they removed the distinction between day and night. That is why Gold and Silver are credited with introducing the day-night cycle.