diff --git a/man/org.freedesktop.login1.xml b/man/org.freedesktop.login1.xml
index bb1ee0b337d..9362f7d8732 100644
--- a/man/org.freedesktop.login1.xml
+++ b/man/org.freedesktop.login1.xml
@@ -736,6 +736,12 @@ node /org/freedesktop/login1 {
       <function>CancelScheduledShutdown()</function> cancels a scheduled shutdown. The output parameter
       <varname>cancelled</varname> is true if a shutdown operation was scheduled.</para>
 
+      <para>Note that <varname>usec</varname> is a lower bound only: the shutdown operation is executed as soon
+      as possible, but not before the specified time. A timestamp in the past is not an error, it simply means
+      the operation is executed immediately. This is important, since a request to shut down immediately, or a
+      very short time into the future, typically already refers to the past by the time it is
+      processed.</para>
+
       <para><function>SetWallMessage()</function> sets the wall message (the message that will be sent out to
       all terminals and stored in a
       <citerefentry project="man-pages"><refentrytitle>utmp</refentrytitle><manvolnum>5</manvolnum></citerefentry> record) for a
diff --git a/man/shutdown.xml b/man/shutdown.xml
index b92e5836f50..46507f6b845 100644
--- a/man/shutdown.xml
+++ b/man/shutdown.xml
@@ -49,6 +49,13 @@
     for triggering an immediate shutdown. If no time argument is
     specified, <literal>+1</literal> is implied.</para>
 
+    <para>The time string specifies the earliest point in time the shutdown may take place; it is a lower
+    bound only: the shutdown is executed as soon as possible, but not before the specified time. Specifying a
+    time that already elapsed (for example an <literal>hh:mm</literal> time earlier today) is not an error and
+    results in an immediate shutdown. See the discussion of <option>--when=</option> in
+    <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
+    details.</para>
+
     <para>Note that to specify a wall message you must specify a time
     argument, too.</para>
 
diff --git a/man/systemctl.xml b/man/systemctl.xml
index 8b64b179ea5..e02fd22534d 100644
--- a/man/systemctl.xml
+++ b/man/systemctl.xml
@@ -2953,6 +2953,19 @@ Jan 12 10:46:45 example.com bluetoothd[8900]: gatt-time-server: Input/output err
           <literal>auto</literal> will schedule the action according to maintenance window or one minute in
           the future.</para>
 
+          <para>The specified timestamp is a lower bound only: it declares the earliest point in time the
+          requested action may take place, i.e. the action is executed as soon as possible, but not before the
+          given timestamp. It is <emphasis>not</emphasis> a condition that is checked, and specifying a
+          timestamp in the past is not an error: the action is then executed immediately. This is intentional,
+          as the alternative would be racy and surprising — consider <option>--when=now</option>, which by the
+          time the request is processed necessarily refers to the past, or
+          <option>--when=+50ms</option>, which might well already have elapsed due to scheduling latencies
+          before the request is dispatched. In both cases the requested action is still carried out, as
+          the user asked for it.</para>
+
+          <para>If a timestamp in the past shall not result in the action being executed, this must be checked
+          for explicitly before invoking <command>systemctl</command>.</para>
+
           <xi:include href="version-info.xml" xpointer="v254"/>
         </listitem>
       </varlistentry>