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).

ntpdate is a simple NTP client which allows a system's clock to be set
to match the time obtained by communicating with one or more servers.

ntpdate is optional (but recommended) if you're running an NTP server,
because initially setting the system clock to an almost-correct time
will help the NTP server synchronize faster.

The ntpdate client by itself is useful for occasionally setting the time on
machines that are not on the net full-time, such as laptops.