Correcting CentOS System Clock

Richard asked:

Using the time C function (number of seconds since the Epoch) shows that the time on my current CentOS 6 server is about 7 hours behind compared to another server with the correct time. How can I correct the system clock? I don’t think it’s drift because I just setup this server a few weeks ago, but it could be. I setup ntpd but it’s not helping, maybe because the time difference is too much.

My answer:


The simple answer is “set the date manually”, which you need to do, but to prevent this occurring again, there is more that you should do.

  1. Ensure that the system timezone configuration is in a sane state.

    Unless there is a very strong reason not to do so (such as software compatibility issues), server clocks should always run on UTC time.

    If you decide not to use UTC, choose a timezone by running tzselect. A timezone will be printed on screen which you will use below. An example would be Europe/Moscow. Otherwise use UTC as the timezone below.

    Here is that TZ value again, this time on standard output so that you
    can use the /usr/bin/tzselect command in shell scripts:
    Europe/Moscow
    

    Set the system clock to your desired timezone by the following steps:

    1. Replace the contents of /etc/sysconfig/clock with the following:

      ZONE="<timezone>"
      UTC=true
      

      For example:

      ZONE="Europe/Moscow"
      UTC=true
      

      Note that UTC=true should be set here, even if you don’t use UTC as your timezone. This refers to the server’s hardware clock, which should always be UTC regardless of your chosen system timezone.

    2. Replace the /etc/localtime file with a link to the selected timezone:

      # ln -snf /usr/share/zoneinfo/<timezone> /etc/localtime
      

      For example:

      # ln -snf /usr/share/zoneinfo/Europe/Moscow /etc/localtime
      # ln -snf /usr/share/zoneinfo/UTC /etc/localtime
      
  2. Set the clock manually to the current time.

    1. Sync the system clock to the current time:

      # ntpd -g -q
      
    2. Check that the time appears correct:

      # date
      
    3. Sync the server’s hardware clock to the system clock:

      # hwclock -wu
      
  3. Restart the computer. Restarting is necessary because all system services must be restarted to pick up the corrected time and timezone, and the server’s hardware clock needs to be tested (e.g. for a faulty battery).

    After restart, check to see that the system shows the correct time and that ntpd is running properly.


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