Real-time clock (RTC)
A tiny timekeeping circuit that tracks date and time even when the rest of the system is off.
Almost every RTC runs from a 32,768 Hz crystal. The number is 215, so a fifteen-stage binary counter yields exactly one tick per second, and the crystal draws so little current that a coin cell lasts for years. The IBM PC/AT got Motorola's MC146818 in 1984, which also kept the BIOS settings in battery-backed memory — which is why the flat battery on a motherboard is still called the CMOS battery.
In microcontrollers the RTC usually sits in its own power domain that survives deep sleep and can wake the CPU at a set time. A cheap crystal drifts a few seconds a week depending on temperature, so connected gadgets correct against NTP, and the unconnected ones show the wrong time, slowly and with total confidence.