NTP is a TCP/IP protocol for synchronising time over a network.
If you don't have such a connection, restrain yourself to ntpdate.If you use modems for dialup, forget ntp and use ntpdate.Ntp *requires* a reliable, fast, permanent internet connection for at least four or five hours a day to work properly AFAIK (but it really wants 24/7 connections), as well as a good configuration and nearby stratum2 time servers (it will work otherwise, yes.But not properly and you'd be better off with only ntpdate). Update Sorry for the bum steer guys, this machine is a Xen Server VM, which I managed to forget. This is called slewing, you can read about it in 'man ntpdate' or RFC 1305.
Generally ntp will slowly adjust the time so when you look at logs things still make sense, and applications don't get messed up.Behind this simple description, there is a lot of complexity - there are tiers of NTP servers, with the tier one NTP servers connected to atomic clocks, and tier two and three servers spreading the load of actually handling requests across the Internet.Also the client software is a lot more complex than you might think - it has to factor out communication delays, and adjust the time in a way that does not upset all the other processes that run on the server. That shall ensure that no two time syncing services are fighting and also to retain any kind of old behaviour/config that you had through an upgrade.Note: make sure your timezone is set properly, which this tool can do as well.ntp and ntpdate and installed both (i have an always on connection to the ntpdate is used to do a "one time only" update to your clock.One disk to bring them all and in the darkness grind them.