HTP!

Firewalled off from NTP? HTP to the rescue!

HTP is not really a protocol, but uses a feature from HTTP, aka web traffic. According the specifications of HTTP (RFC 2616) a web server needs to put a timestamp in a response to a web browser request. In web browsers you don't see the HTTP headers, but these headers contain a timestamp in Greenwich Mean Time (GMT), accurate in seconds.

Available in Perl or C. My compliments to Eddy Vervest.