You're on a PC connected to a domain, and you want the PC to have its clock set to the proper time automatically, without your intervention.
Using a graphical user interface
Figure 6-1. If you're connected to a domain, the Windows Time service will automatically connect to a domain controller to synchronize its clock
Using a command-line interface
At the command line, type net start w32time and press Enter. That will start the Windows Time service. To stop the service, at the command line type net stop w32time and press Enter. That will stop the Windows Time service. When you use the command line to start the Windows Time service, the service only runs for that single session. If you want the service to start every time you start XP you can use the service's management module, or you can use the sc.exe utility (run sc /config for more information).
Only computers running Windows XP Professional can connect to domains, so XP Professional has Windows Time service, while the XP Home Edition does not. By default, the Windows Time service runs automatically on Windows XP Professional computers.
Time synchronization takes place with the Windows Time service during system startup. During startup, the Net Logon service looks for a domain controller that can synchronize time with the PC. It follows the Active Directory hierarchy. When the service finds a domain controller, it sends a request for time and waits for a reply. The communication is an exchange of SNTP packets that calculates the time offset and the roundtrip delay between the two computers.
You can also use the Registry to synchronize a PC's time with a domain controller on stsartup. Open the Registry Editor, and go to the key HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\Parameters. Create a REG_SZ value called Type and give it the value Nt5DS. That will tell the computer to synchronize its time with a domain controller on startup. If you give it the value NoSync, it will tell the computer not to synchronize its time with a domain controller.
For more details about using the Windows Time service, see http://www.microsoft.com/technet/prodtechnol/winxppro/maintain/xpmanaged/27_xpwts.mspx. For more in-formation about using the Registry and the Windows Time service, see http://www.winguides.com/registry/display.php/1118.