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.

src/core/timer.c

@@ -95,6 +95,7 @@ static int timer_verify(Timer *t) {
 
 static int timer_add_default_dependencies(Timer *t) {
         int r;
+        TimerValue *v;
 
         assert(t);
 
@@ -106,6 +107,15 @@ static int timer_add_default_dependencies(Timer *t) {
                 r = unit_add_two_dependencies_by_name(UNIT(t), UNIT_AFTER, UNIT_REQUIRES, SPECIAL_SYSINIT_TARGET, NULL, true);
                 if (r < 0)
                         return r;
+
+                LIST_FOREACH(value, v, t->values) {
+                        if (v->base == TIMER_CALENDAR) {
+                                r = unit_add_dependency_by_name(UNIT(t), UNIT_AFTER, SPECIAL_TIME_SYNC_TARGET, NULL, true);
+                                if (r < 0)
+                                        return r;
+                                break;
+                        }
+                }
         }
 
         return unit_add_two_dependencies_by_name(UNIT(t), UNIT_BEFORE, UNIT_CONFLICTS, SPECIAL_SHUTDOWN_TARGET, NULL, true);