I use a RPi 3 as a secondary DNS and DHCP server, and time synchronization is important for that. Due to some technicalities with how my network is set up, this means that I need a real-time clock on the RPi so that it can have at least some idea of the correct time when it powers up instead of being absolutely dependant on NTP for that.
Enter the DS3231 RTC (available on eBay for a few bucks). The Pi Hut has an excellent tutorial on setting this up for a RPi, which I’m going to summarize here.
Configure I2C on the RPi
From a root shell (I’m assuming you’re using Raspbian like me);
apt-get install python-smbus apt-get install i2c-tools
Then, edit your /boot/config.txt and add the following down the bottom;
dtparam=i2c_arm=on dtoverlay=i2c-rtc,ds3231
Edit your /etc/modules and add the following line;
i2c-dev
Now reboot. If you do an i2cdetect -y 1 you should see the DS3231 listed as device 0x68. If you do, great.
Configure Raspbian to use the RTC
After rebooting, the new device should be up, but you won’t be using it yet. Remove the fake hardware clock with;
apt-get --purge remove fake-hwclock
Now you should be able do hwclock -r to read the clock, and then hwclock-w to write the current time to it.
And lastly, to make it pull time from the RTC on boot, put the following into /etc/rc.local before the exit 0;
hwclock -s
And you can then add a cronjob in /etc/cron.weekly to run hwclock -w once a week.
Done!