The DS3231 is a low-cost, extremely accurate I²C real-time clock (RTC) with an integrated temperature-compensated crystal oscillator (TCXO) and crystal.

# For Ubuntu 22.04, and any other distro that doesn’t have usercfg.txt:

1. Install the DS3231 module to the Raspberry Pi’s GPIO.
2. Open the config.txt file located on the boot partition. (/boot/firmware/config.txt inside Raspberry Pi or D:\config.txt (drive letter might be different) in Windows.)
3. Add the following text below the bottommost [all] to your config.txt: dtoverlay=i2c-rtc,ds3231.
4. Test the RTC by entering this command sudo hwclock -r. A time string should appear.

# For Ubuntu 20.04, and any other distro that have usercfg.txt:

1. Install the DS3231 module to the Raspberry Pi’s GPIO.
2. Open the usercfg.txt file located on the boot partition. (/boot/firmware/usercfg.txt inside Raspberry Pi or D:\usercfg.txt (drive letter might be different) in Windows.)
3. Add the following text to your usercfg.txt: dtoverlay=i2c-rtc,ds3231.
4. Test the RTC by entering this command sudo hwclock -r. A time string should appear.

# For Raspberry Pi OS (formerly Raspbian):

1. Install the DS3231 module to the Raspberry Pi’s GPIO.
2. Open the config.txt file located on the boot partition. (/boot/config.txt inside Raspberry Pi or D:\config.txt (drive letter might be different) in Windows.)
3. Uncomment the dtparam=i2c_arm=on on the config.txt by removing the # in front of the line. Or use raspi-config to enable I²C.
4. Add the following text below the [all] to your config.txt: dtoverlay=i2c-rtc,ds3231.
5. Test the RTC by entering this command sudo hwclock -r. A time string should appear.

Tested on Raspberry Pi 4 Model B.

Source: Install DS3231 Real Time Clock - Latest Info