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systemd/man/systemd.time.xml
Lennart Poettering 3c719357dc man: extend on the usec/sec discrepancy 
Let's document the discrepancy between the Sec and USec suffixing of
unit files and D-Bus properties at three places: in "systemctl show"
(where it already was briefly mentioned), in the D-Bus interface
description (at one place at least, i.e. the most prominent of
properties that encapsulate time values, there are many more) and in the
general man page explaining time values.

By documenting this at all three places I think we now do as much as we
can do about this highlighting the discrepancy of the naming and the
reasons behind it.

Fixes: #2047
2020-08-28 18:01:17 +02:00

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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+ -->
<refentry id="systemd.time">
<refentryinfo>
<title>systemd.time</title>
<productname>systemd</productname>
</refentryinfo>
<refmeta>
<refentrytitle>systemd.time</refentrytitle>
<manvolnum>7</manvolnum>
</refmeta>
<refnamediv>
<refname>systemd.time</refname>
<refpurpose>Time and date specifications</refpurpose>
</refnamediv>
<refsect1>
<title>Description</title>
<para>In systemd, timestamps, time spans, and calendar events are
displayed and may be specified in closely related syntaxes.</para>
</refsect1>
<refsect1>
<title>Displaying Time Spans</title>
<para>Time spans refer to time durations. On display, systemd will present time spans as a space-separated series
of time values each suffixed by a time unit. Example:</para>
<programlisting>2h 30min</programlisting>
<para>All specified time values are meant to be added up. The above hence refers to 150 minutes. Display is
locale-independent, only English names for the time units are used.</para>
</refsect1>
<refsect1>
<title>Parsing Time Spans</title>
<para>When parsing, systemd will accept the same time span syntax.
Separating spaces may be omitted. The following time units are
understood:</para>
<itemizedlist>
<listitem><para>usec, us, µs</para></listitem>
<listitem><para>msec, ms</para></listitem>
<listitem><para>seconds, second, sec, s</para></listitem>
<listitem><para>minutes, minute, min, m</para></listitem>
<listitem><para>hours, hour, hr, h</para></listitem>
<listitem><para>days, day, d</para></listitem>
<listitem><para>weeks, week, w</para></listitem>
<listitem><para>months, month, M (defined as 30.44 days)</para></listitem>
<listitem><para>years, year, y (defined as 365.25 days)</para></listitem>
</itemizedlist>
<para>If no time unit is specified, generally seconds are assumed, but some exceptions exist and are marked as
such. In a few cases <literal>ns</literal>, <literal>nsec</literal> is accepted too, where the granularity of the
time span permits this. Parsing is generally locale-independent, non-English names for the time units are not
accepted.</para>
<para>Examples for valid time span specifications:</para>
<programlisting>2 h
2hours
48hr
1y 12month
55s500ms
300ms20s 5day</programlisting>
<para>One can use the <command>timespan</command> command of
<citerefentry><refentrytitle>systemd-analyze</refentrytitle><manvolnum>1</manvolnum></citerefentry>
to normalise a textual time span for testing and validation purposes.</para>
<para>Internally, systemd generally operates with microsecond time granularity, while the default time
unit in user-configurable time spans is usually seconds (see above). This disparity becomes visible when
comparing the same settings in the (high-level) unit file syntax with the matching (more low-level) D-Bus
properties (which are what
<citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>'s
<command>show</command> command displays). The former typically are suffixed with <literal>…Sec</literal>
to indicate the default unit of seconds, the latter are typically suffixed with <literal>…USec</literal>
to indicate the underlying low-level time unit, even if they both encapsulate the very same
settings.</para>
</refsect1>
<refsect1>
<title>Displaying Timestamps</title>
<para>Timestamps refer to specific, unique points in time. On
display, systemd will format these in the local timezone as
follows:</para>
<programlisting>Fri 2012-11-23 23:02:15 CET</programlisting>
<para>The weekday is printed in the abbreviated English language form. The formatting is locale-independent.</para>
<para>In some cases timestamps are shown in the UTC timezone instead of the local timezone, which is indicated via
the <literal>UTC</literal> timezone specifier in the output.</para>
<para>In some cases timestamps are shown with microsecond granularity. In this case the sub-second remainder is
separated by a full stop from the seconds component.</para>
</refsect1>
<refsect1>
<title>Parsing Timestamps</title>
<para>When parsing, systemd will accept a similar syntax, but expects no timezone specification, unless
it is given as the literal string <literal>UTC</literal> (for the UTC timezone), or is specified to be
the locally configured timezone, or the timezone name in the IANA timezone database format. The complete
list of timezones supported on your system can be obtained using the <literal>timedatectl
list-timezones</literal> (see
<citerefentry><refentrytitle>timedatectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>). Using
IANA format is recommended over local timezone names, as less prone to errors (e.g. with local timezone
it's possible to specify daylight saving time in winter, even though that is not correct). The weekday
specification is optional, but when the weekday is specified, it must either be in the abbreviated
(<literal>Wed</literal>) or non-abbreviated (<literal>Wednesday</literal>) English language form (case
does not matter), and is not subject to the locale choice of the user. Either the date, or the time part
may be omitted, in which case the current date or 00:00:00, respectively, is assumed. The seconds
component of the time may also be omitted, in which case ":00" is assumed. Year numbers may be specified
in full or may be abbreviated (omitting the century).</para>
<para>A timestamp is considered invalid if a weekday is specified and the date does not match the specified day of
the week.</para>
<para>When parsing, systemd will also accept a few special
placeholders instead of timestamps: <literal>now</literal> may be
used to refer to the current time (or of the invocation of the
command that is currently executed). <literal>today</literal>,
<literal>yesterday</literal>, and <literal>tomorrow</literal> refer to
00:00:00 of the current day, the day before, or the next day,
respectively.</para>
<para>When parsing, systemd will also accept relative time
specifications. A time span (see above) that is prefixed with
<literal>+</literal> is evaluated to the current time plus the
specified time span. Correspondingly, a time span that is prefixed
with <literal>-</literal> is evaluated to the current time minus
the specified time span. Instead of prefixing the time span with
<literal>+</literal> or <literal>-</literal>, it may also be
suffixed with a space and the word <literal>left</literal> or
<literal>ago</literal>.</para>
<para>Finally, a timespan prefixed with <literal>@</literal> is
evaluated relative to the UNIX time epoch 1st Jan, 1970,
00:00.</para>
<para>Examples for valid timestamps and their normalized form (assuming the current time was 2012-11-23
18:15:22 and the timezone was UTC+8, for example <literal>TZ=:Asia/Shanghai</literal>):</para>
<programlisting> Fri 2012-11-23 11:12:13 → Fri 2012-11-23 11:12:13
2012-11-23 11:12:13 → Fri 2012-11-23 11:12:13
2012-11-23 11:12:13 UTC → Fri 2012-11-23 19:12:13
2012-11-23 → Fri 2012-11-23 00:00:00
12-11-23 → Fri 2012-11-23 00:00:00
11:12:13 → Fri 2012-11-23 11:12:13
11:12 → Fri 2012-11-23 11:12:00
now → Fri 2012-11-23 18:15:22
today → Fri 2012-11-23 00:00:00
today UTC → Fri 2012-11-23 16:00:00
yesterday → Fri 2012-11-22 00:00:00
tomorrow → Fri 2012-11-24 00:00:00
tomorrow Pacific/Auckland → Thu 2012-11-23 19:00:00
+3h30min → Fri 2012-11-23 21:45:22
-5s → Fri 2012-11-23 18:15:17
11min ago → Fri 2012-11-23 18:04:22
@1395716396 → Tue 2014-03-25 03:59:56</programlisting>
<para>Note that timestamps displayed by remote systems with a non-matching timezone are usually not parsable
locally, as the timezone component is not understood (unless it happens to be <literal>UTC</literal>).</para>
<para>Timestamps may also be specified with microsecond granularity. The sub-second remainder is expected separated
by a full stop from the seconds component. Example:</para>
<programlisting>2014-03-25 03:59:56.654563</programlisting>
<para>In some cases, systemd will display a relative timestamp (relative to the current time, or the time of
invocation of the command) instead of or in addition to an absolute timestamp as described above. A relative
timestamp is formatted as follows:</para>
<programlisting>2 months 5 days ago</programlisting>
<para>Note that a relative timestamp is also accepted where a timestamp is expected (see above).</para>
<para>Use the <command>timestamp</command> command of
<citerefentry><refentrytitle>systemd-analyze</refentrytitle><manvolnum>1</manvolnum></citerefentry> to
validate and normalize timestamps for testing purposes.</para>
</refsect1>
<refsect1>
<title>Calendar Events</title>
<para>Calendar events may be used to refer to one or more points
in time in a single expression. They form a superset of the
absolute timestamps explained above:</para>
<programlisting>Thu,Fri 2012-*-1,5 11:12:13</programlisting>
<para>The above refers to 11:12:13 of the first or fifth day of
any month of the year 2012, but only if that day is a Thursday or
Friday.</para>
<para>The weekday specification is optional. If specified, it
should consist of one or more English language weekday names,
either in the abbreviated (Wed) or non-abbreviated (Wednesday)
form (case does not matter), separated by commas. Specifying two
weekdays separated by <literal>..</literal> refers to a range of
continuous weekdays. <literal>,</literal> and <literal>..</literal>
may be combined freely.</para>
<para>In the date and time specifications, any component may be specified as <literal>*</literal> in
which case any value will match. Alternatively, each component can be specified as a list of values
separated by commas. Values may be suffixed with <literal>/</literal> and a repetition value, which
indicates that the value itself and the value plus all multiples of the repetition value are matched.
Two values separated by <literal>..</literal> may be used to indicate a range of values; ranges may also
be followed with <literal>/</literal> and a repetition value, in which case the expression matches all
times starting with the start value, and continuing with all multiples of the repetition value relative
to the start value, ending at the end value the latest.</para>
<para>A date specification may use <literal>~</literal> to indicate the
last day(s) in a month. For example, <literal>*-02~03</literal> means
"the third last day in February," and <literal>Mon *-05~07/1</literal>
means "the last Monday in May."</para>
<para>The seconds component may contain decimal fractions both in
the value and the repetition. All fractions are rounded to 6
decimal places.</para>
<para>Either time or date specification may be omitted, in which
case the current day and 00:00:00 is implied, respectively. If the
second component is not specified, <literal>:00</literal> is
assumed.</para>
<para>Timezone can be specified as the literal string <literal>UTC</literal>, or
the local timezone, similar to the supported syntax of timestamps (see above), or the timezone
in the IANA timezone database format (also see above).</para>
<para>The following special expressions may be used as shorthands for longer normalized forms:</para>
<programlisting> minutely → *-*-* *:*:00
hourly → *-*-* *:00:00
daily → *-*-* 00:00:00
monthly → *-*-01 00:00:00
weekly → Mon *-*-* 00:00:00
yearly → *-01-01 00:00:00
quarterly → *-01,04,07,10-01 00:00:00
semiannually → *-01,07-01 00:00:00
</programlisting>
<para>Examples for valid timestamps and their
normalized form:</para>
<programlisting> Sat,Thu,Mon..Wed,Sat..Sun → Mon..Thu,Sat,Sun *-*-* 00:00:00
Mon,Sun 12-*-* 2,1:23 → Mon,Sun 2012-*-* 01,02:23:00
Wed *-1 → Wed *-*-01 00:00:00
Wed..Wed,Wed *-1 → Wed *-*-01 00:00:00
Wed, 17:48 → Wed *-*-* 17:48:00
Wed..Sat,Tue 12-10-15 1:2:3 → Tue..Sat 2012-10-15 01:02:03
*-*-7 0:0:0 → *-*-07 00:00:00
10-15 → *-10-15 00:00:00
monday *-12-* 17:00 → Mon *-12-* 17:00:00
Mon,Fri *-*-3,1,2 *:30:45 → Mon,Fri *-*-01,02,03 *:30:45
12,14,13,12:20,10,30 → *-*-* 12,13,14:10,20,30:00
12..14:10,20,30 → *-*-* 12..14:10,20,30:00
mon,fri *-1/2-1,3 *:30:45 → Mon,Fri *-01/2-01,03 *:30:45
03-05 08:05:40 → *-03-05 08:05:40
08:05:40 → *-*-* 08:05:40
05:40 → *-*-* 05:40:00
Sat,Sun 12-05 08:05:40 → Sat,Sun *-12-05 08:05:40
Sat,Sun 08:05:40 → Sat,Sun *-*-* 08:05:40
2003-03-05 05:40 → 2003-03-05 05:40:00
05:40:23.4200004/3.1700005 → *-*-* 05:40:23.420000/3.170001
2003-02..04-05 → 2003-02..04-05 00:00:00
2003-03-05 05:40 UTC → 2003-03-05 05:40:00 UTC
2003-03-05 → 2003-03-05 00:00:00
03-05 → *-03-05 00:00:00
hourly → *-*-* *:00:00
daily → *-*-* 00:00:00
daily UTC → *-*-* 00:00:00 UTC
monthly → *-*-01 00:00:00
weekly → Mon *-*-* 00:00:00
weekly Pacific/Auckland → Mon *-*-* 00:00:00 Pacific/Auckland
yearly → *-01-01 00:00:00
annually → *-01-01 00:00:00
*:2/3 → *-*-* *:02/3:00</programlisting>
<para>Calendar events are used by timer units, see
<citerefentry><refentrytitle>systemd.timer</refentrytitle><manvolnum>5</manvolnum></citerefentry>
for details.</para>
<para>Use the <command>calendar</command> command of
<citerefentry><refentrytitle>systemd-analyze</refentrytitle><manvolnum>1</manvolnum></citerefentry> to validate
and normalize calendar time specifications for testing purposes. The tool also calculates when a specified
calendar event would occur next.</para>
</refsect1>
<refsect1>
<title>See Also</title>
<para>
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.timer</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd-analyze</refentrytitle><manvolnum>1</manvolnum></citerefentry>
</para>
</refsect1>
</refentry>
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