chrony is a client and server for the Network Time Protocol (NTP). This program keeps your computer's clock accurate. It was specially designed to support systems with intermittent Internet connections, but it also works well in permanently connected environments. It can also use hardware reference clocks, the system real-time clock, or manual input as time references.
ntpdate is a simple and small replacement for the homonymous and obsolete tool from the NTP Project's (www.ntp.org) reference implementation. ntpdate sets the local date and time by polling the Network Time Protocol (NTP) server(s) given as the server argument to determine the correct time. It must be run as root on the local host. It is not meant for real time synchronization.
OmniSync is a driver for NTPd for people who are firewall-challenged. It enables systems to synchronize time when port 123 (UDP) is blocked. It allows you to sync against the daytime service, time, SNTS, IRC, ICMP, SNMP, precision time protocol (PTP -- IEEE 1588), HTTP, HTTPS (both also via proxy server), and (S)NTP via a Socks(5) proxy server. It doesn't directly set the clock, but uses NTPd for this, as this enables you to have multiple time sources as well as bad-chimer detection, etc.