How to Setup a Windows 2008/2012 R2 NTP Server

Here's how to do it:

1.    Click Start, click Run, type regedit, and then click OK.

2.    Locate and then click the following registry entry:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\Config\

3.    In the right pane, right-click AnnounceFlags, and then click Modify.

4.    In the Edit DWORD Value dialog box, under Value data, type 5, and then click OK.

5.    Enable NTPServer.

1.    Locate and then click the following registry subkey:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
                                                Services\W32Time\TimeProviders\NtpServer\

2.    In the right pane, right-click Enabled, and then click Modify.

3.    In the Edit DWORD Value dialog box, type 1 under Value data, and then click OK.

6.    Exit Registry Editor.

7.    At the command prompt, type the following command to restart the Windows Time service, and then press ENTER:

net stop w32time && net start w32time

XenServer : 

#Service ntpd Stop