I’ve encountered a problem with a freshly installed Ubuntu 18.04. The time was 2 minutes ahead. I tried ntp/ntpd/openntp and none could correct the time.
It’s after wasting about 1 hour that I questioned whether common NTP servers are blocked. I googled it, and there is a blog saying that cloudclone blocked NTP port 123., and I should use ntp.cloudcone.com instead.
So it’s best if you can update your linux images or install scripts to configure ntp.cloudcone.com as default, so that new comers won’t waste time on this again.