DS3231 RTC

The DS3231 RTC is a low cost, battery backup, Real Time Clock, that connects to an I2C bus, providing accurate time information.

Many DS3231 modules, like this one, https://www.ebay.com/itm/314250880191, have two I2C devices on-board,
a DS3231 RTC and a AT24C32 EEPROM, allowing the storage of information into non-volatile memory.

Documents:
DS3231: https://www.analog.com/media/en/technical-documentation/data-sheets/ds3231.pdf
The time (and temperature), are accessed via 19 byte-wide registers, see Address Map on page 11.

AT24C32: https://ww1.microchip.com/downloads/en/devicedoc/doc0336.pdf
This 32Kbit (4096 x 8), will hold 4096 bytes of data.

NOTE!
These modules typically have a battery charging circuit to recharge a LIR2032, lithium coin cell.
This can cause a problem if using CR2032, a cheaper non-rechargable coin cell.
This page describes the problem:
  https://forum.arduino.cc/t/ds3231-cr2032-vs-lir2032-warning-is-your-module-killing-the-battery/557346
  My preferred fix is to desolder the diode (or resistor), and lift one end from the board, thus
  defeating the charging circuit.  This method allows the circuit to be restored easily in the future.