Real time clocks serve to time tracking. Clock is connected to a 3.3V battery that keeps the circuit in a state that allows clock to work. Other features are off. Once the power is connected, the microcontroller can read the current state with I2C or set a new time.
Both circuits are also available in the form of modules that facilitate their use on breadboard. The EEPROM memory and the 3.3V battery holder are also usually included on the modules.