The DS3231 is a real-time clock (RTC), with an integrated temperature compensated crystal oscillator and crystal.
The DS3231 has a fixed I2C address of 0x68, and contains 19 indexed registers. The first 7 registers are for time and date keeping, followed by 7 registers for two alarms, 3 registers for status and control of the chip, and 2 registers for temperature. It's the continous monitoring of temperature that allows this chip to be exceedingly accurate. It compensates its oscillator circuit based on the current temperature.