Questions regarding Real Time Clocks (RTCs) which can be a timekeeper IC used by microcontrollers, or using a microcontroller as an accurate timekeeper. Some have calendar, alarm, interrupt or heartbeat features, while some simply keep track of time since last programmed (Epoch timers are similar).
RTCs can be external to the microcontroller, or integrated on the same die. RTCs typically run off a 32.768kHz tuning fork crystal and are low-power, typically less than 1\$\mu\$A. On the microcontroller integrated RTCs have their own power supply, independent of the rest of the microcontroller.
RTCs have a set of date/time registers which are kept by the crystal oscillator and are readable by the microcontroller, most often via a serial interface when in a separate IC. Other features are heartbeat interrupts and alarm interrupts at settable times. RTC IC often have a small amount of battery backup RAM.
Further reading
Real-time clocks/calendars plus literature list at NXP
