Can I use NTP service through a proxy?

NTP uses UDP packets with port number 123. So if you are behind aproxy or firewall, and the proxy or firewall blocks the UDP port 123, the public NTP servers you specify in /etc/ntp.conf won't be reachable to your host. If you can't reach them then you need to use internal ntp servers if they are available on your network.

If internal NTP servers (with their own clock source; e.g. GPS) and dual-homed NTP servers are not an option, then a lower accuracy option is htpdate (Note: not ntpdate). This pulls time from the timestamp inside a HTTP header - so it is not the most accurate.

Note: this system presumes that the web servers used as a time source have accurate time

See http://www.clevervest.com/htp/