Man Linux: Main Page and Category List


       chrony - programs for keeping computer clocks accurate


       chronyc [OPTIONS]

       chronyd [OPTIONS]


       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.


       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
       accordingly.  It also works out the rate  at  which  the  system  clock
       gains  or  loses  time  and  uses  this information to keep it accurate
       between measurements 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.  Of course, it will work
       on computers with permanent connections too.

       In addition, for Linux 2.0.x (for x >= 32) or 2.2 onwards, chronyd  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.


       chronyc(1), chrony(1)


       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.