The following KB article covers setting up (and troubleshooting) the ability of Windows to sync with an external NTP time source:
In the troubleshooting section, it says:
For the Windows Time service to function correctly, the networking infrastructure must function correctly. The most common problems that affect the Windows Time service include the following:
- There is a problem with TCP/IP
connectivity, such as a dead gateway.
- The Name Resolution service is not
- The network is
experiencing high volume delays,
especially when synchronization
occurs over high-latency wide area
network (WAN) links.
- The Windows Time
service is trying to synchronize with
inaccurate time sources.