ntp-doc 1:4.2.8p10+dfsg-5ubuntu4 (amd64 binary) in ubuntu bionic

 NTP, the Network Time Protocol, is used to keep computer clocks
 accurate by synchronizing them over the Internet or a local network,
 or by following an accurate hardware receiver that interprets GPS,
 DCF-77, NIST or similar time signals.
 .
 This package contains HTML documentation for the ntp packages (ntp,
 ntpdate). Since there are no substantive man pages for ntp's
 programs and configuration files, this package is desirable for any
 setup beyond the simple default configuration.