This is the original, complete, documentation for NTP, in HTML format.
Manpages documentation comes with the binary package(s).

The Network Time Protocol (NTP) is used to synchronize a computer's time
with another reference time source. The ntp package contains utilities
and daemons which will synchronize your computer's time to Coordinated
Universal Time (UTC) via the NTP protocol and NTP servers. Ntp includes
ntpdate (a program for retrieving the date and time from remote machines
via a network) and ntpd (a daemon which continuously adjusts system time).