timer: order OnCalendar units after timer-sync.target if DefaultDependencies=no

Avoids triggering timers prematurely on systems with significantly inaccurate
clocks, or some embedded platforms that lack one entirely.

man/systemd.timer.xml

@@ -78,15 +78,20 @@
                 <varname>Unit=</varname> (see below).</para>
 
                 <para>Unless <varname>DefaultDependencies=</varname>
-                is set to <option>false</option>, timer units will
+                is set to <option>false</option>, all timer units will
                 implicitly have dependencies of type
                 <varname>Conflicts=</varname> and
                 <varname>Before=</varname> on
-                <filename>shutdown.target</filename>. These ensure
-                that timer units are stopped cleanly prior to system
-                shutdown. Only timer units involved with early boot or
-                late system shutdown should disable this
-                option.</para>
+                <filename>shutdown.target</filename> to ensure that
+                they are stopped cleanly prior to system shutdown.
+                Timer units with at least one
+                <varname>OnCalendar=</varname> directive will have an
+                additional <varname>After=</varname> dependency on
+                <filename>timer-sync.target</filename> to avoid
+                being started before the system clock has been
+                correctly set. Only timer units involved with early
+                boot or late system shutdown should disable the
+                <varname>DefaultDependencies=</varname> option.</para>
         </refsect1>
 
         <refsect1>