Here's a question that keeps coming up that I keep forgetting the answer to:
Say you're trying to measure how long it takes some data to get from one computer to another, whether it's an RPC call or a message or whatever. Or say you want to use message expiration. In either case, to measure the elapsed time, both computers have to agree on what time it currently is. How do you make sure that two computers' clocks are synchronized?
The answer (as you may have guessed from the title) is the Network Time Protocol (NTP). It runs on Internet Protocol (IP), typically on port 123. It uses a master/slave configuration: the master hosts a reference clock and the NTP service to make that clock available; the slaves synchronize by getting the time from the master and using that to set their own clocks.
For more information:
BTW, a handy dandy site for showing the time to us humans (at least in the United States) is The Official U.S. Time, operated by the National Institute of Standards and Technology (NIST) and the U. S. Naval Observatory (USNO). It displays Coordinated Universal Time (UTC) for any U.S. time zone you choose, including adjustments for Daylight Saving Time when appropriate.
Did you know you can configure Windows XP to automatically synchronize your computer's clock with an Internet time server? See Synchronizing your computer clock.