Using and configuring the RTC module

In config:

dtoverlay=i2c-rtc,ds3231
dtparam=i2c_arm=on

Reboot and then check it in the terminal that the i2c device is available:

sudo i2cdetect -l|sort
sudo i2cdetect -y 1 

Next is the setup:

sudo apt-get -y remove fake-hwclock
sudo update-rc.d -f fake-hwclock remove
sudo systemctl disable fake-hwclock

Edit the file hwclock-set:

sudo nano /lib/udev/hwclock-set

Complete file:

#!/bin/sh
# Reset the System Clock to UTC if the hardware clock from which it
# was copied by the kernel was in localtime.

dev=$1

#if [ -e /run/systemd/system ] ; then
#    exit 0
#fi

if [ -e /run/udev/hwclock-set ]; then
    exit 0
fi

if [ -f /etc/default/rcS ] ; then
    . /etc/default/rcS
fi

# These defaults are user-overridable in /etc/default/hwclock
BADYEAR=no
HWCLOCKACCESS=yes
HWCLOCKPARS=
HCTOSYS_DEVICE=rtc0
if [ -f /etc/default/hwclock ] ; then
    . /etc/default/hwclock
fi

if [ yes = "$BADYEAR" ] ; then
#    /sbin/hwclock --rtc=$dev --systz --badyear
    /sbin/hwclock --rtc=$dev --hctosys --badyear
else
#    /sbin/hwclock --rtc=$dev --systz
    /sbin/hwclock --rtc=$dev --hctosys
fi

# Note 'touch' may not be available in initramfs
> /run/udev/hwclock-set

Reboot and check in terminal:

timedatectl status

Read:

sudo hwclock -r

Write:

sudo hwclock -w

Additional info

sudo hwclock --verbose