Hacker's ramblings

I don't have a Windows-domain at home, so the Internet time client (NTP) is on relaxed settings. Your typical Microsoft documentation about NTP will have phrases like: "The default value for domain members is 10. The default value for stand-alone clients and servers is 15" in it. So, it really makes a difference if the computer is in a domain or not.

It is a well established fact, that the hardware clock on your computer is quite inaccurate. On a modern computer, there is no point in using expensive hardware to make the clock run smoothly, you can always set the time from a reliable clock source from Internet. That's what the NTP was made decades ago, to make sure that everybody has the same time in their boxes.

The real question here is: Why does my Windows 7 clock skew so much? I have set up the internet time, but it still is inaccurate.

As a Linux-guy I love doing my stuff on the command-line. To question about the clock skew I'll do:

w32tm /monitor /computers:-the-NTP-server-

... and it will respond something like NTP: -0.7900288s offset from local clock. So it's almost a second behind the accurate time source.

The initial fix is easy, force it to get the accurate time from the configured time server:

w32tm /resync

But I cannot be doing that all the time. Why cannot the computer maintain a well disciplined clock like I configured it to do? There must be something fishy about that.

A command like:

w32tm /query /status

will say that Poll Interval: 10 (1024s), but I cannot confirm that requests for every 1024 seconds (or less). It simply does not do that. There is a TechNet article with the title of Windows Time Service Tools and Settings describing a registry setting of MaxPollInterval located in HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\Config, but that has no real relevance here. The update mechanism does not obey that setting.

However, Microsoft's knowledge base article 884776 titled How to configure the Windows Time service against a large time offset gives more insight about the update interval. It describes a registry value of SpecialPollInterval located in HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\TimeProviders\NtpClient for manual peers. I'm guessing I have a manual peer, whatever that means. I don't have a domain and I did set the server manually. The original value seems to be 604800 seconds, making that 7 days or a week. Whoa! Way too much for me.

While sniffing the network traffic with the Wireshark, indeed I can confirm that putting a small value into that will make my Windows 7 to poll on that interval. I put 10 seconds there, and it seems to work. For any real life scenario 10 seconds to update time is ridiculous. For a computer on a domain, the value is 3600 seconds, making the updates for every hour. I chose to use that.

Please note that changing the registry value requires a restart for the Windows time client. From a command line a:

net stop w32time
net start w32time

will do the trick and start using the newly set registry value. You can also restart the Windows Time service from GUI.

Now my computer's time seems to stick with a reasonable accuracy. I'm still considering of purchasing a GPS-time box of my own. They seem to be quite expensive, though.