How does the clock work in Pokémon Gold and Silver?

Gold and Silver cartridges carry an MBC3 controller with a real-time clock, powered by the cartridge's battery. The player sets the time and the day of the week when the journey begins, and from then on the cartridge keeps time even while it sits in a drawer.

The day divides into three periods rather than a smooth cycle: morning from 4:00 AM to 9:59 AM, day from 10:00 AM to 5:59 PM, and night from 6:00 PM to 3:59 AM. The world answers visually, with a yellowish tint in the morning and a darkened blue-purple night in which buildings light their windows.

Time is content. Different Pokémon appear in each period, berries regrow on their trees, the Week Siblings each appear on their own day to hand over items, and the Bug-Catching Contest at the National Park runs only on Tuesdays, Thursdays and Saturdays.

The clock is also the cartridge's weak point. Because the battery maintains the clock continuously, it drains faster than a save-only battery, and a cartridge that can no longer keep time is a cartridge whose save is living on borrowed time, the subject of the next shelf in this cluster.