- <para>The first argument should be the string
- <command>now</command> which however is ignored by
- this implementation. Optionally, this may be followed
- by a wall message to be sent to all logged-in users
- before going down.</para>
+ <para>The first argument may be a time string (which
+ is usually <literal>now</literal>). Optionally, this
+ may be followed by a wall message to be sent to all
+ logged-in users before going down.</para>
+
+ <para>The time string may either be in the format
+ <literal>hh:mm</literal> for hour/minutes specifying
+ the time to execute the shutdown at, specified in 24h
+ clock format. Alternatively it may be in the syntax
+ <literal>+m</literal> referring to the specified
+ number of minutes m from now. <literal>now</literal>
+ is an alias for <literal>+0</literal>, i.e. for
+ triggering an immediate shutdown. If no time argument
+ is specified, <literal>now</literal> is
+ implied.</para>
+
+ <para>Note that to specify a wall message you must
+ speciy a time argument, too.</para>
+
+ <para>If the time argument is used, 5 minutes
+ before the system goes down the
+ <filename>/etc/nologin</filename> file is created to
+ ensure that further logins shall not be
+ allowed.</para>