I'm having some problems manually setting my PC's time from the Internet. I'm using Windows 8, but have had the same problems with another PC on Windows 7. I've tried the built in functionality of Windows and with Dimension 4 Software to set the time. Most of the time I cannot connect to the Time Server. I have tried several time servers including time.nist.gov and time.windows.com
NTP protocol utilizes the User Datagram Protocol (UDP) on port 123. Check your firewall & make sure you can reach a time server on port 123.
If it turns out your ISP is blocking port 123, another option is a local GPS clock connected to one of your computers via a USB/serial port.
Parallax makes a unit with a native serial port (SKU 28500), that retails for about $40.00
73 Martin Flynn