The animated GIF clock program ("Content-type: multipart/x-mixed-replace"). clock technology creates an animated GIF image on the fly, and uses server-push technology to display on a Netscape browser. This technology enabled precise control over when the time changes occur at the browser. This is done by starting the running seconds at the top of the next occurring second, and pushing new GIFs out each second. The success of this technique has established it as the USNO's longest-running web application, with over 160 million connections logged.

Microsoft chose not to support server-push technology in its Internet Explorer (as of May 2003). We successfully have tested Netscape 7 for this application. The open- sourced Mozilla browser also supports server-push technology. However, because of the resource load that server-push imposes on our server, USNO does not guarantee to continue to offer this service in the future and reserves the right to discontinue the demonstration at any time. In addition, as browsers become increasingly complex their utilization of caching and input buffering makes real-time displaying less and less feasible.

The animated GIF clocks are offered only as a demonstration. Other clock displays on the Department web site, such as this link, are supported by Internet Explorer.

For actual time synchronization USNO supports only the Internet RFC Standard Network Time Protocol (NTP) available from www.ntp.org.