I remember looking at this circa 2016 - 2021 and letting go at that time, crony had just been released and systemd was still relatively new at that time.

• sudo systemctl status systemd-timesyncd
  • The default configuration is shown in /etc/systemd/timesyncd.conf (All the defaults are shown commented out)
  • Copy the default configuration in /etc/systemd/timesyncd.conf to /etc/systemd/timesyncd.conf.d/timesyncd.conf and modify options in /etc/systemd/timesyncd.conf.d/timesyncd.conf
• timedatectl timesync-status

ntp servers can be checked using the ntpdate command, which can be loaded in Debian with sudo apt install ntpsec-ntpdate

List of local ntp servers

• STRATUMS IN THE NTP PROTOCOL: Understanding the hierarchy of servers
• chrony
• cturra/ntp (crony docker example)
• How to Install and Use Chrony in Linux
• configuring-ntp-server
• Arch chrony
• Steps to configure Chrony as NTP Server & Client (CentOS/RHEL 8)
• How to serve the Network Time Protocol with Chrony
• The Essential Guide to Using Chrony for NTP on Linux