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chrony(1) [centos man page]

CHRONY(1)							   User's Manual							 CHRONY(1)

chrony - programs for keeping computer clocks accurate SYNOPSIS
chrony is a pair of programs for keeping computer clocks accurate. chronyd is a background (daemon) program and chronyc is a command-line interface to it. Time reference sources for chronyd can be RFC1305 NTP servers, human (via keyboard and chronyc), or the computer's real- time clock at boot time (Linux only). chronyd can determine the rate at which the computer gains or loses time and compensate for it while no external reference is present. Its use of NTP servers can be switched on and off (through chronyc) to support computers with dial- up/intermittent access to the Internet, and it can also act as an RFC1305-compatible NTP server. USAGE
chronyc is a command-line interface program which can be used to monitor chronyd's performance and to change various operating parameters whilst it is running. chronyd's main function is to obtain measurements of the true (UTC) time from one of several sources, and correct the system clock accord- ingly. It also works out the rate at which the system clock gains or loses time and uses this information to keep it accurate between mea- surements from the reference. The reference time can be derived from either Network Time Protocol (NTP) servers, reference clocks, or wristwatch-and-keyboard (via chronyc). The main source of information about the Network Time Protocol is It is designed so that it can work on computers which only have intermittent access to reference sources, for example computers which use a dial-up account to access the Internet or laptops. Of course, it will work well on computers with permanent connections too. In addition, on Linux it can monitor the system's real time clock performance, so the system can maintain accurate time even across reboots. Typical accuracies available between 2 machines are On an ethernet LAN : 100-200 microseconds, often much better On a V32bis dial-up modem connection : 10's of milliseconds (from one session to the next) With a good reference clock the accuracy can reach one microsecond. chronyd can also operate as an RFC1305-compatible NTP server and peer. SEE ALSO
chronyc(1), chrony(1) AUTHOR
Richard Curnow <> This man-page was written by Jan Schaumann <> as part of "The Missing Man Pages Project". Please see for details. The complete chrony documentation is supplied in texinfo format. chrony @VERSION@ @MAN_DATE@ CHRONY(1)

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TIMEDATECTL(1)							    timedatectl 						    TIMEDATECTL(1)

timedatectl - Control the system time and date SYNOPSIS
timedatectl may be used to query and change the system clock and its settings. OPTIONS
The following options are understood: -h, --help Prints a short help text and exits. --version Prints a short version string and exits. --no-pager Do not pipe output into a pager. --no-ask-password Do not query the user for authentication for privileged operations. -P, --privileged Acquire privileges via PolicyKit before executing the operation. -H, --host Execute the operation remotely. Specify a hostname, or username and hostname separated by "@", to connect to. This will use SSH to talk to a remote system. --adjust-system-clock If set-local-rtc is invoked and this option is passed, the system clock is synchronized from the RTC again, taking the new setting into account. Otherwise, the RTC is synchronized from the system clock. The following commands are understood: status Show current settings of the system clock and RTC. set-time [TIME] Set the system clock to the specified time. This will also update the RTC time accordingly. The time may be specified in the format "2012-10-30 18:17:16". set-timezone [TIMEZONE] Set the system time zone to the specified value. Available timezones can be listed with list-timezones. If the RTC is configured to be in the local time, this will also update the RTC time. This call will alter the /etc/localtime symlink. See localtime(5) for more information. list-timezones List available time zones, one per line. Entries from the list can be set as the system timezone with set-timezone. set-local-rtc [BOOL] Takes a boolean argument. If "0", the system is configured to maintain the RTC in universal time. If "1", it will maintain the RTC in local time instead. Note that maintaining the RTC in the local timezone is not fully supported and will create various problems with time zone changes and daylight saving adjustments. If at all possible, keep the RTC in UTC mode. Note that invoking this will also synchronize the RTC from the system clock, unless --adjust-system-clock is passed (see above). This command will change the 3rd line of /etc/adjtime, as documented in hwclock(8). set-ntp [BOOL] Takes a boolean argument. Controls whether NTP based network time synchronization is enabled (if available). EXIT STATUS
On success, 0 is returned, a non-zero failure code otherwise. ENVIRONMENT
$SYSTEMD_PAGER Pager to use when --no-pager is not given; overrides $PAGER. Setting this to an empty string or the value "cat" is equivalent to passing --no-pager. EXAMPLES
Show current settings: $ timedatectl Local time: Fri, 2012-11-02 09:26:46 CET Universal time: Fri, 2012-11-02 08:26:46 UTC RTC time: Fri, 2012-11-02 08:26:45 Timezone: Europe/Warsaw UTC offset: +0100 NTP enabled: no NTP synchronized: no RTC in local TZ: no DST active: no Last DST change: CEST -> CET, DST became inactive Sun, 2012-10-28 02:59:59 CEST Sun, 2012-10-28 02:00:00 CET Next DST change: CET -> CEST, DST will become active the clock will jump one hour forward Sun, 2013-03-31 01:59:59 CET Sun, 2013-03-31 03:00:00 CEST Enable an NTP daemon (chronyd): $ timedatectl set-ntp true ==== AUTHENTICATING FOR org.freedesktop.timedate1.set-ntp === Authentication is required to control whether network time synchronization shall be enabled. Authenticating as: user Password: ******** ==== AUTHENTICATION COMPLETE === $ systemctl status chronyd.service chronyd.service - NTP client/server Loaded: loaded (/usr/lib/systemd/system/chronyd.service; enabled) Active: active (running) since Fri, 2012-11-02 09:36:25 CET; 5s ago ... SEE ALSO
systemd(1), hwclock(8), date(1), localtime(5), systemctl(1), systemd-timedated.service(8) systemd 208 TIMEDATECTL(1)
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