Creating a clock using Arduino
If you are willing to go the “barebones” route, you can turn an
ATmega328P (the MCU at the heart of the Arduino Uno) into a very
accurate RTC by hooking a watch crystal to it's clock pins. Given proper
calibration, you can get an accuracy of a few seconds per year.

This would require some coding though, and is not as straightforward as
using an off-the-shelf RTC. C.f.
this answer for the details.