This article shows how to use a Raspberry Pi on Ubuntu Mate with a real time clock to create a NTP server.

https://thepihut.com/products/adafruit-pirtc-pcf8523-real-time-clock-for-raspberry-pi

1. Power Off Raspberry Pi
2. Install PFC8523 on I/O Pins and install the battery
3. Power Up Raspberry Pi
4. After login run sudo raspi-config and enable i2c interface
5. Edit /boot/config.txt and add the following to the end of the filedtoverlay=i2c-rtc,pcf8523
6. After updating the /boot/config.txt, reboot the Raspberry Pi
7. Manually set the data/time on the Raspberry Pi, then use

   hwclock --systohc

   to set the time on the pfc8523

8. Confirm the pfc8523 is working correctly by using

   hwclock --show

In order to get the Raspberry Pi to read the RTC and keep the pfc8523 correctly synchronised follow the instructions below

Edit the file /etc/systemd/timesyncd.conf with the following entries:

[Time]
  NTP=0.europe.pool.ntp.org 1.europe.pool.ntp.org 2.europe.pool.ntp.org 3.europe.pool.ntp.org
  FallbackNTP=0.europe.pool.ntp.org 1.europe.pool.ntp.org 2.europe.pool.ntp.org 3.europe.pool.ntp.org

Update the file /etc/rc.local by adding the following lines:

[ ! -e /var/lib/systemd/clock -a "`systemctl is-active systemd-timesyncd | grep -i active`" ] && timedatectl set-ntp 1 > /dev/null 2>&1

Create a file /etc/systemd/system/hwclock-sync.service with the following contents:

[Unit]
  Description=Time Synchronisation from RTC Source
  After=systemd-modules-load.service
  RequiresMountsFor=/dev/rtc
  Conflicts=shutdown.target
  [Service]
  Type=oneshot
  ExecStart=/sbin/hwclock -s
  TimeoutSec=0
  [Install]
  WantedBy=time-sync.target

Now we need to shutdown and disable services for ntp and fake-hwclock and remove the corresponding packages that come by default with the OS by executing the following set of commands on the “root” shell prompt:

systemctl stop fake-hwclock

systemctl disable fake-hwclock prefer-timesyncd.service systemd-timedated.service

apt-get -y remove fake-hwclock

To finally enable the automatic start-up of the RTC synchonrisation during bootup using the systemd-timesyncd service, execute the following commands on the shell prompt:

systemctl enable hwclock-sync systemd-timesyncd

systemctl start hwclock-sync systemd-timesyncd

apt-get install ntp

Edit /etc/ntp.conf

server 0.europe.pool.ntp.org
  server 1.europe.pool.ntp.org
  server 2.europe.pool.ntp.org
  server 3.europe.pool.ntp.org

Run the following commands to activate ntp

sudo systemctl enable ntp

sudo systemctl start ntp

After five minutes, run ntpq and use the command peers to show the clocks ntp is synchronised with

ntpq> peers

ntpq> exit

Now verify correct operation by using the

timedatectl

command to obtain results similar to below

Local time: Sun 2017-10-15 12:13:55 BST
  Universal time: Sun 2017-10-15 11:13:55 UTC
  RTC time: Sun 2017-10-15 11:13:55
  Time zone: Europe/London (BST, +0100)
  Network time on: yes
  NTP synchronized: yes
  RTC in local TZ: no