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

Ranked by testing with sudo ntpdate -q {ntp servername/ip}