sudo raspi-config
Go to "9 Advanced Options" / "A7 I2C" and enable it including the module load etc
2. sudo reboot
3. Add the following line at the end of /boot/config.txt in Raspbian Jessy:
dtoverlay=i2c-rtc,ds3231
4. We don’t need fake-hwclock anymore:
sudo apt-get purge fake-hwclock
5. After updating time from NTP server and it is correct , write time to rtc clock
sudo hwclock -w
6.sudo reboot
7. Edit /etc/rc.local and add the hwclock command above the line that says “exit 0”:
/sbin/hwclock -s
sudo nano /etc/rc.local
8. edit /etc/init.d/hwclock.sh to HWCLOCKACCESS=no
sudo nano /etc/init.d/hwclock.sh
9. check the clock module is detected
dmesg | grep rtc
10. Compare rtc and system time
sudo hwclock -c