Reading computer time stamp over internet

Hello SuSE folkz, I went to this web site: http://greenwichmeantime.com/gmt-timestamp.htm and saw that they somehow read the time stamp right from my PC. I'm behind the firewall and all ports are in stealth mode. I wonder, how they do this. Is it a part of TCP/IP protocol or environmental variable of my browser? Thanks in advance for any replies. Alex