It sounds like the computer thinks it is part of a Domain. As you mentioned the internet time tab is disabled for domain environments. The computer will sync with the domain controller and there isn’t a way to have it sync to the internet in domains.
I think this article from Microsoft might help. It provides a number of registry values that effect this area.
Also, you might be able to run a few commands from command prompt to resolve this too:
w32tm /config /syncfromflags:MANUAL /manualpeerlist:time.nist.gov w32tm /config /update
If your computer is not a domain member, the Windows Time service can be set to Manual startup mode and should ordinarily not be running.
Try just stopping the service to see if the additional tab returns. You may have to go as far as setting it to Manual startup and rebooting.
Additionally, the registry key
HKEY\Software\Microsoft\Windows\CurrentVersion\DateTime\Servers needs to be configured correctly in order to make it work. In the screen shot below, there are six available time servers listed, but the (Default) value is empty.
The (Default) value needs to have something in it. Otherwise, the Internet Time tab will not show. Specifically, the (Default) value needs to have the name of one of the other values in this Servers key. It cannot have an invalid value. Using the screen shot again, a value of 6 would be invalid, because there is no corresponding time server labelled “6.” On my machine, the (Default) value is 1, which means my computer gets time from time.windows.com.
In previous versions of Windows, the Internet Time tab was not built-into the Date and Time applet,
but was added there by the Windows Time service. In Windows 7 I found weird things happening,
such as seeing the service running although it was set to Manual, or of seeing Internet Time displayed and time-sync done even when the service is disabled.
I’m starting to think that Windows 7 will start the service when time-sync is to be done,
no matter what is specified as its Startup type. I still don’t know whether the Internet Time tab
is now built-in or is still being added there by the service on its first run.
It seems however likely to me that there is still some connection between the service
and the Internet Time tab, so that some corruption caused this mechanism to malfunction.
The Time service is defined in the registry at
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\services\W32Time. In case that the problem is caused by registry corruption, I have posted this key’s contents on pastebin for comparison. Do not import it blindly into the registry (or at least not before saving yours).
Otherwise, all I can suggest are general troubleshooting steps such as the System File Checker.
My computer is part of a Domain and the tab is not visible. If I disable the network card, then the tab can be seen/accessed. Re-enabling the NIC turns the tab invisible again.