I have some droplets on DigitalOcean, but I noted that the system time (in all of them) is not correctly synced with NTP servers.

The solution that worked for me is to include the following line at the end of /etc/crontab:

* * * * * root /sbin/hwclock -s

That command instructs the system to copy time from hardware (which is more precise) every minute. That solved my problem.