ntpdate(8) System Manager’s Manual ntpdate(8)


ntpdate – set the date and time via NTP


ntpdate [-bBdoqsuv] [-a key] [-e authdelay] [-k keyfile] [-o version] [-p samples] [-t

timeout] server […]


ntpdate sets the local date and time by polling the Network Time Protocol (NTP) server(s)

given as the server arguments to determine the correct time. It must be run as root on

the local host (unless the option -q is used). A number of samples are obtained from each

of the servers specified and a subset of the NTP clock filter and selection algorithms

are applied to select the best of these. Note that the accuracy and reliability of ntp‐

date depends on the number of servers, the number of polls each time it is run and the

interval between runs.

ntpdate can be run manually as necessary to set the host clock, or it can be run from the

host startup script to set the clock at boot time. This is useful in some cases to set

the clock initially before starting the NTP daemon ntpd. It is also possible to run ntp‐

date from a cron script. However, it is important to note that ntpdate with contrived

cron scripts is no substitute for the NTP daemon, which uses sophisticated algorithms to

maximize accuracy and reliability while minimizing resource use. Finally, since ntpdate

does not discipline the host clock frequency as does ntpd, the accuracy using ntpdate is


Time adjustments are made by ntpdate in one of two ways. If ntpdate determines the clock

is in error more than 0.5 second it will simply step the time by calling the system set‐

timeofday() routine. If the error is less than 0.5 seconds, it will slew the time by

calling the system adjtime() routine. The latter technique is less disruptive and more

accurate when the error is small, and works quite well when ntpdate is run by cron every

hour or two.

ntpdate will decline to set the date if an NTP server daemon (e.g., ntpd) is running on

the same host. When running ntpdate on a regular basis from cron as an alternative to

running a daemon, doing so once every hour or two will result in precise enough timekeep‐

ing to avoid stepping the clock.


-a key Enable the authentication function and specify the key identifier to be used for
authentication as the argument keyntpdate. The keys and key identifiers must match

in both the client and server key files. The default is to disable the authentica‐

tion function.

-B Force the time to always be slewed using the adjtime() system call, even if the
measured offset is greater than +-128 ms. The default is to step the time using

settimeofday() if the offset is greater than +-128 ms. Note that, if the offset is

much greater than +-128 ms in this case, that it can take a long time (hours) to

slew the clock to the correct value. During this time, the host should not be used

to synchronize clients.

-b Force the time to be stepped using the settimeofday() system call, rather than
slewed (default) using the adjtime() system call. This option should be used when

called from a startup file at boot time.

-d Enable the debugging mode, in which ntpdate will go through all the steps, but not
adjust the local clock. Information useful for general debugging will also be


-e authdelay
Specify the processing delay to perform an authentication function as the value

authdelay, in seconds and fraction (see ntpd for details). This number is usually

small enough to be negligible for most purposes, though specifying a value may

improve timekeeping on very slow CPU’s.

-k keyfile
Specify the path for the authentication key file as the string keyfile. The

default is /etc/ntp.keys. This file should be in the format described in ntpd.

-o version
Specify the NTP version for outgoing packets as the integer version, which can be

1 or 2. The default is 3. This allows ntpdate to be used with older NTP versions.

-p samples
Specify the number of samples to be acquired from each server as the integer sam‐

ples, with values from 1 to 8 inclusive. The default is 4.

-q Query only – don’t set the clock.

-s Divert logging output from the standard output (default) to the system syslog
facility. This is designed primarily for convenience of cron scripts.

-t timeout
Specify the maximum time waiting for a server response as the value timeout, in

seconds and fraction. The value is is rounded to a multiple of 0.2 seconds. The

default is 1 second, a value suitable for polling across a LAN.

-u Direct ntpdate to use an unprivileged port for outgoing packets. This is most
useful when behind a firewall that blocks incoming traffic to privileged ports,

and you want to synchronise with hosts beyond the firewall. Note that the -d

option always uses unprivileged ports.

-v Be verbose. This option will cause ntpdate’s version identification string to be


ntpdate’s exit status is zero if it found a server and could update the clock, and

nonzero otherwise.


– encryption keys used by ntpdate.


The slew adjustment is actually 50% larger than the measured offset, since this (it is

argued) will tend to keep a badly drifting clock more accurate. This is probably not a

good idea and may cause a troubling hunt for some values of the kernel variables tick and



David L. Mills (

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