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+<?xml version='1.0'?> <!--*-nxml-*-->
+<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
+ "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
+<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
+
+<refentry id="systemd-timesyncd.service" conditional='ENABLE_TIMESYNCD'
+ xmlns:xi="http://www.w3.org/2001/XInclude">
+
+ <refentryinfo>
+ <title>systemd-timesyncd.service</title>
+ <productname>systemd</productname>
+ </refentryinfo>
+
+ <refmeta>
+ <refentrytitle>systemd-timesyncd.service</refentrytitle>
+ <manvolnum>8</manvolnum>
+ </refmeta>
+
+ <refnamediv>
+ <refname>systemd-timesyncd.service</refname>
+ <refname>systemd-timesyncd</refname>
+ <refpurpose>Network Time Synchronization</refpurpose>
+ </refnamediv>
+
+ <refsynopsisdiv>
+ <para><filename>systemd-timesyncd.service</filename></para>
+ <para><filename>/usr/lib/systemd/systemd-timesyncd</filename></para>
+ </refsynopsisdiv>
+
+ <refsect1>
+ <title>Description</title>
+
+ <para><filename>systemd-timesyncd</filename> is a system service that may be used to synchronize the
+ local system clock with a remote Network Time Protocol (NTP) server. It also saves the local time to disk
+ every time the clock has been synchronized and uses this to possibly advance the system realtime clock on
+ subsequent reboots to ensure it (roughly) monotonically advances even if the system lacks a
+ battery-buffered RTC chip.</para>
+
+ <para>The <filename>systemd-timesyncd</filename> service implements SNTP only. This minimalistic service
+ will step the system clock for large offsets or slowly adjust it for smaller deltas. Complex use cases
+ that require full NTP support (and where SNTP is not sufficient) are not covered by
+ <filename>systemd-timesyncd</filename>.</para>
+
+ <para>The NTP servers contacted are determined from the global settings in
+ <citerefentry><refentrytitle>timesyncd.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>, the
+ per-link static settings in <filename>.network</filename> files, and the per-link dynamic settings
+ received over DHCP. See
+ <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
+ further details.</para>
+
+ <para><citerefentry><refentrytitle>timedatectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>'s
+ <command>set-ntp</command> command may be used to enable and start, or disable and stop this
+ service.</para>
+
+ <para><citerefentry><refentrytitle>timedatectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>'s
+ <command>timesync-status</command> or <command>show-timesync</command> command can be used to show the
+ current status of this service.</para>
+
+ <para><filename>systemd-timesyncd</filename> initialization delays the start of units that are ordered
+ after <filename>time-set.target</filename> (see
+ <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry> for
+ details) until the local time has been updated from <filename>/var/lib/systemd/timesync/clock</filename>
+ (see below) in order to make it roughly monotonic. It does not delay other units until synchronization
+ with an accurate reference time sources has been reached. Use
+ <citerefentry><refentrytitle>systemd-time-wait-sync.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+ to achieve that, which will delay start of units that are ordered after
+ <filename>time-sync.target</filename> until synchronization to an accurate reference clock is
+ reached.</para>
+ </refsect1>
+
+ <refsect1>
+ <title>Files</title>
+
+ <variablelist>
+ <varlistentry>
+ <term><filename>/var/lib/systemd/timesync/clock</filename></term>
+
+ <listitem>
+ <para>The modification time ("mtime") of this file is updated on each successful NTP
+ synchronization or after each <varname>SaveIntervalSec=</varname> time interval, as specified in
+ <citerefentry><refentrytitle>timesyncd.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
+
+ <para>When initializing, the local clock is advanced to the modification time of this file (if the
+ file timestamp is in the past this adjustment is not made). If the file does not exist yet, the
+ clock is instead advanced to the modification time of <filename>/usr/lib/clock-epoch</filename> –
+ if it exists – or to a time derived from the source tree at build time. This mechanism is used to
+ ensure that the system clock remains somewhat reasonably initialized and roughly monotonic across
+ reboots, in case no battery-buffered local RTC is available.</para>
+
+ <xi:include href="version-info.xml" xpointer="v219"/>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><filename>/usr/lib/clock-epoch</filename></term>
+
+ <listitem><para>The modification time ("mtime") of this file is used for advancing the system clock
+ in case <filename>/var/lib/systemd/timesync/clock</filename> does not exist yet, see
+ above.</para>
+
+ <xi:include href="version-info.xml" xpointer="v254"/></listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><filename>/run/systemd/timesync/synchronized</filename></term>
+
+ <listitem>
+ <para>A file that is touched on each successful synchronization, to assist
+ <filename>systemd-time-wait-sync</filename> and other applications to detecting synchronization
+ with accurate reference clocks.</para>
+
+ <xi:include href="version-info.xml" xpointer="v239"/>
+ </listitem>
+
+ </varlistentry>
+ </variablelist>
+ </refsect1>
+
+ <refsect1>
+ <title>See Also</title>
+ <para>
+ <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
+ <citerefentry><refentrytitle>timesyncd.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
+ <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
+ <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
+ <citerefentry><refentrytitle>systemd-time-wait-sync.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
+ <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
+ <citerefentry><refentrytitle>timedatectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
+ <citerefentry><refentrytitle>localtime</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
+ <citerefentry project='man-pages'><refentrytitle>hwclock</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+ </para>
+ </refsect1>
+
+</refentry>