To be more specific :
Upto R6 you cannot use NTP servers, the time source on Pre 7 system is either a VM Pro server or a PC with a running Manager or a TIME server.
If neither one can be used set the time IP Address to 0.0.0.1 and configure a phone as a "System" phone. A system phone can change the date and time setting.
program a button with the function "Time"
On later systems R7+ you can use a NTP server to synchronize the time, any NTP server will do but most Domain Controllers not as these only reward NTP requests from known Domain PCs. Most local networks have a NTP server somewere, the default gateway, the DHCP server (if not a Domain controller) or if the IP Office has internet access the Internet provider. Like NTP.MyProviderHasIt.com or ntp.windows.com or 0.us.pool.ntp.org as suggested, I prefer a local time server.
If there is no NTP server available/reachable then set the Time server to None and set one or more phones with sytem permission 1.
The newer phones like 1408/1416/1616/95xx and 96xx have the option to change the date and time in the phone menu. For older types like the 24xx/54xx/46xx you need to program a button with the function "Time".
Hope that helps