[jerdfelt]

There is NTP the protocol[1], and there is NTP the implementation[2].

While the implementation is popular, there are alternatives. There is also OpenNTPd, chrony and ntimed for instance.

There are also alternatives to the NTP protocol too, such as PTP and SNTP.

[1]https://www.ietf.org/rfc/rfc5905.txt [2]http://www.ntp.org/

[tatersolid]

Don't forget the billion-plus machines out there running Windows Time Service (which strangely has had zero security issues I can remember, even when running in server mode).

[JdeBP]

W32Time has different kinds of issues, in my experience. With it, one's problems tend to be that, by design until very recently, it doesn't provide to-the-second accuracy.

* https://blogs.technet.microsoft.com/askds/2007/10/23/high-ac...

* https://greyware.com/software/domaintime/v5/overview/w32time...

* https://technet.microsoft.com/en-gb/windows-server-docs/iden...

[JdeBP]

... and TAICLOCK.

* http://cr.yp.to/proto/taiclock.txt