Home site of the xntpd time server client and deamon.
Source is provided.
It uses one time passwords in a challenge responce system that you need to use a calculation program to provide the responce. Individuals using this system without authority or in excess of their authority are subject to having all their activities on this system monitored and recorded or examined by any authorized person, including law enforcement, as system personnel deem appropriate. In the course of monitoring individuals improperly using the system or in the course of system maintenance, the activities of authorized users may also be monitored and recorded. Any material so recorded may be disclosed as appropriate. Anyone using this system consents to these terms.