set-amazon-ntp.sh

#!/bin/bash
# run as root, then validate with:
# chronyc sources -v
# chronyc tracking
# see https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/set-time.html#configure-time-sync

apt install -y chrony
sed -i '/\# See http:\/\/www.pool.ntp.org\/join.html for more information./a server 169.254.169.123 prefer iburst minpoll 4 maxpoll 4\npool time.aws.com iburst' /etc/chrony/chrony.conf
systemctl enable --now chrony
/etc/init.d/chrony restart

Instead of start, do systemctl enable --now chrony.

Updated, thanks @verdimrc