diff --git a/sched/sched/sched_addreadytorun.c b/sched/sched/sched_addreadytorun.c
index 2724a1bd5acb7..c22571df8a183 100644
--- a/sched/sched/sched_addreadytorun.c
+++ b/sched/sched/sched_addreadytorun.c
@@ -103,6 +103,17 @@ bool nxsched_add_readytorun(FAR struct tcb_s *btcb)
       btcb->flink->task_state = TSTATE_TASK_READYTORUN;
       up_update_task(btcb);
       ret = true;
+
+#if defined(CONFIG_SCHED_TICKLESS) && CONFIG_RR_INTERVAL > 0
+      /* Tickless has no periodic tick to keep RR accounting alive, so
+       * arm the new task's timeslice timer explicitly on switch-in.
+       */
+
+      if ((btcb->flags & TCB_FLAG_POLICY_MASK) == TCB_FLAG_SCHED_RR)
+        {
+          nxsched_reassess_timer();
+        }
+#endif
     }
   else
     {
diff --git a/sched/wdog/wdog.h b/sched/wdog/wdog.h
index bd726af57d791..c52a986dbc2db 100644
--- a/sched/wdog/wdog.h
+++ b/sched/wdog/wdog.h
@@ -176,7 +176,14 @@ static inline_function void wd_timer_start(clock_t tick, bool in_expiration)
 #ifdef CONFIG_SCHED_TICKLESS_ALARM
   up_alarm_tick_start(next_tick);
 #else
-  up_timer_tick_start(next_tick - clock_systime_ticks());
+  clock_t delta = next_tick - clock_systime_ticks();
+
+  /* Clamp to >= 1 tick: an already-expired 'delta' can go negative
+   * (overflows unsigned in arch code), and even zero can miss a
+   * oneshot timer's rising-edge match if the target is already due.
+   */
+
+  up_timer_tick_start(delta > 0 ? delta : 1);
 #endif
 }