DS3231 RTC Module

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 charger circuit to recharge a lithium coin cell.
       This can cause a problem is using non-rechargable coin cells.