First, the Jetson should have a supercap backed real-time clock (if you’re using the standard motherboard.)
If this doesn’t work, your supercap may be broken.
If you’re using another carrier board, check with the documentation for whether you need to hook up a battery or a supercap.
Second, starting a service doesn’t mean that it will stay running across reboot. Ubuntu is a systemd release, and you need to enable the service. And, on ubuntu, it’s called “systemd-timesyncd.service” not “ntpd.”
sudo systemctl enable systemd-timesyncd.service
However, this should already be enabled. You can check the status for whether it is running correctly:
systemctl status systemd-timesyncd.service
Chances are, the problem you’re seeing is with the real-time clock or your carrier board, and not with the time synchronizer.