lfs-book/chapter07/clock.xml

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<sect1 id="ch-scripts-clock" revision="systemd">
  <?dbhtml filename="clock.html"?>

  <title>Configuring the system clock</title>

  <indexterm zone="ch-scripts-clock">
    <primary sortas="d-clock">clock</primary>
  <secondary>configuring</secondary></indexterm>

  <para>This section discusses how to configure the
  <command>systemd-timedated</command> system service, which configures
  system clock and timezone.</para>

  <para>If you cannot remember whether or not the hardware clock is set to UTC,
  find out by running the <userinput>hwclock --localtime --show</userinput>
  command. This will display what the current time is according to the hardware
  clock. If this time matches whatever your watch says, then the hardware clock is
  set to local time. If the output from <command>hwclock</command> is not local
  time, chances are it is set to UTC time. Verify this by adding or subtracting
  the proper amount of hours for the timezone to the time shown by
  <command>hwclock</command>. For example, if you are currently in the MST
  timezone, which is also known as GMT -0700, add seven hours to the local
  time.</para>

  <para><command>systemd-timedated</command> reads <filename>/etc/adjtime</filename>,
  and depending on the contents of the file, it sets the clock to either UTC or
  local time.</para>

  <para>Create the <filename>/etc/adjtime</filename> file with the following contents
  if your hardware clock is set to local time:</para>

<screen><userinput>cat &gt; /etc/adjtime &lt;&lt; "EOF"
<literal>0.0 0 0.0
0
LOCAL</literal>
EOF</userinput></screen>

  <para>If <filename>/etc/adjtime</filename> isn't present at first boot,
  <command>systemd-timedated</command> will assume that hardware clock is
  set to UTC and adjust the file according to that.</para>

  <para>You can also use the <command>timedatectl</command> utility to tell
  <command>systemd-timedated</command> if your hardware clock is set to
  UTC or local time:</para>

<screen role="nodump"><userinput>timedatectl set-local-rtc 1</userinput></screen>

  <para><command>timedatectl</command> can also be used to change system time and
  time zone.</para>

  <para>To change your current system time, issue:</para>

<screen role="nodump"><userinput>timedatectl set-time YYYY-MM-DD HH:MM:SS</userinput></screen>

  <para>Hardware clock will also be updated accordingly.</para>

  <para>To change your current time zone, issue:</para>

<screen role="nodump"><userinput>timedatectl set-timezone TIMEZONE</userinput></screen>

  <para>You can get a list of available time zones by running:</para>

<screen role="nodump"><userinput>timedatectl list-timezones</userinput></screen>

  <note><para>Please note that the <command>timedatectl</command> command can
  be used only on a system booted with systemd.</para></note>

  <sect2>
    <title>Network Time Synchronization</title>

    <para>Starting with version 213, systemd ships a daemon called
    <command>systemd-timesyncd</command> which can be used to
    synchronize the system time with remote NTP servers.</para>

    <para>The daemon is not intended as a replacement for the well
    established NTP daemon, but as a client only implementation
    of the SNTP protocol which can be used for less advanced
    tasks and on resource limited systems.</para>

    <para>Starting with systemd version 216, the
    <command>systemd-timesyncd</command> daemon is enabled by
    default. If you want to disable it, issue the following
    command:</para>

<screen role="nodump"><userinput>systemctl disable systemd-timesyncd</userinput></screen>

    <para>The <filename>/etc/systemd/timesyncd.conf</filename> file
    can be used to change the NTP servers that
    <command>systemd-timesyncd</command> synchronizes with.</para>

    <para>Please note that when system clock is set to Local Time,
    <command>systemd-timesyncd</command> won't update hardware
    clock.</para>

  </sect2>

</sect1>