Computers are really bad at being clocks. Windows computers doubly so. I recently had reason to learn far more than I ever wanted to know about Microsoft's implementation of NTP (which is - I can only assume deliberately - incompatible with the real thing, to the effect that you can sync windows to normal time servers but not vice-versa).
Amongst the lessons learned was not to trust that GUI setting with a barge pole, and to configurationise the Windows time service using the w32tm command[1]. Another lesson was that time.windows.com (which, to be fair, does appear to be a proper time server) has previous for sending out the wrong time. Indeed, this is why when obtaining time from Other People's Computers, unisex spaceadmins tend to configure their hosts to poll more than one time server, so that it can form a consensus and ignore any that are obviously drunk and should go home. You can do that in Windows, but it means fucking about at the command line, rather than editing a text file like dog intended.
[1] A stunning example of how Windows still isn't ready for the desktop.