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OpenNTPD is a FREE, easy to use implementation of the Network Time Protocol. It provides the ability to sync the local clock to remote NTP servers and can act as NTP server itself, redistributing the local clock.
OpenNTPD was primarily developed by Henning Brauer as part of the OpenBSD Project and gets released as a base component of OpenBSD every six months.
The portable version is maintained by Brent Cook and is mirrored on Github and available as periodic tarball releases. Contributions are welcome to both the OpenNTPD core and the portable build framework.