It depends on how accurate you want your system clock to be. Issues:

1. On what time stratum is the server in question running?
2. To what resolution is the timestamp on HTTP headers set by the server software?

If I were on a system where the ntp ports were blocked by the firewall (a rather strange thing to do), I would acquire a cheap GPS module via eBay and set up my own Stratum 1 time server. As a matter of fact, I am doing this anyway, despite the fact that I'm running at Stratum 3 quite happily here.