Datasheet

This is the best battery-backed real time clock (RTC) you can get that
allows your Raspberry Pi project to keep track of time if the power is lost.
Perfect for data-logging, clock-building, NTP servers, time-stamping,
timers and alarms, etc. Equipped with a genuine DS3231 RTC, it works
great with the Raspberry Pi and has native kernel support.
The DS3231is an extremely accurate I²C-Integrated RTC/TCXO/Crystal.
Most RTCs use an external 32kHz timing crystal that is used to keep time
with low current draw, but those crystals have slight drift, particularly
when the temperature changes (the temperature changes the oscillation
frequency very slightly). This RTC is in a thick package because the
crystal is inside the chip! And right next to the integrated crystal is a
temperature sensor. That sensor compensates for the frequency changes
by adding or removing clock ticks so that the timekeeping stays on
schedule.