http://support.microsoft.com/default.aspx?scid=kb;EN-US;223184

In an Active Directory domain-based network, this should only have to be done on the PDC.

To configure a Windows 2000 Server for Network Time Protocol (NTP), enter the following commands as an administrator:

net time /setsntp:pool.ntp.org
net stop w32time
w32tm -once
net start w32time

Windows Server 2003 Forest Root Domain Controller servers are configured like this:

W32tm /config /manualpeerlist:pool.ntp.org /syncfromflags:manual
W32tm /config /update
W32tm /resync

The default sync interval for the Windows Time Service is three times a day. If that isn't good enough, you can adjust it…but it takes a registry edit.

net time /domain:yourdomainname /set /yes

Locate the domain controller where you will configure network time:

netdom /query fsmo

Then, on the indicated domain controller:

W32tm /config /manualpeerlist:0.us.pool.ntp.org,1.us.pool.ntp.org,2.us.pool.ntp.org /syncfromflags:manual /reliable:yes
W32tm /resync

If your time is off by too much (more than 3600 seconds), you'll have to set the time manually with the Date and Time applet.

w32tm /query /configuration

Check the Event Viewer for any error messages.

If for some reason you need to clear the configuration or start over:

net stop w32time
w32tm /unregister
w32tm /register
net start w32time

http://nefaria.com/2013/03/configure-windows-server-20082012-to-sync-with-internet-time-servers/

net stop w32time
w32tm /config /manualpeerlist:"0.us.pool.ntp.org 1.us.pool.ntp.org" /syncfromflags:manual /reliable:yes
net start w32time
w32tm /query /status
w32tm /query /configuration

The most accurate way to keep guest operating system time synchronized with real time is probably to use the VMware Tools or XenServer Tools time synchronization functions.

http://www.virtualbox.org/manual/ch09.html#idp13775264

Install the guest additions or use NTP.

To enable VMware Tools time synchronization in the guest:

1. Right-click the VMware icon in the System tray on the task bar and choose Open VMware Tools.
2. On the Options tab, check the Time synchronization between the virtual machine and the host operating system box.
3. Click OK to close the dialog box.
4. Disable the Windows Time service from the Services control panel in the guest.
   1. Right-click My Computer and choose Manage.
   2. Double-click Services and Applications.
   3. Double-click Services.
   4. Scroll to Windows Time; right-click and choose Properties.
   5. Click the Startup type selection box and choose Disable.
   6. If the service status is Started, click Stop.
   7. Click OK to close the Properties dialog box.

It's a good idea to run a time synchronization tool on the virtualization host, for best accuracy.

 0.us.pool.ntp.org 
 1.us.pool.ntp.org
 2.us.pool.ntp.org
 3.us.pool.ntp.org

Open UDP port 123 on the firewall.