diff --git a/drivers/timer/npcx_itim_timer.c b/drivers/timer/npcx_itim_timer.c
index eb0436c8a6e..bb2734d3fe8 100644
--- a/drivers/timer/npcx_itim_timer.c
+++ b/drivers/timer/npcx_itim_timer.c
@@ -140,8 +140,6 @@ static inline void npcx_itim_evt_disable(void)
 /* ITIM local functions */
 static int npcx_itim_start_evt_tmr_by_tick(int32_t ticks)
 {
-	k_spinlock_key_t key = k_spin_lock(&lock);
-
 	/*
 	 * Get desired cycles of event timer from the requested ticks which
 	 * round up to next tick boundary.
@@ -152,7 +150,7 @@ static int npcx_itim_start_evt_tmr_by_tick(int32_t ticks)
 	} else {
 		uint64_t next_cycs;
 		uint64_t curr = npcx_itim_get_sys_cyc64();
-		uint32_t dcycles;
+		uint64_t dcycles;
 
 		if (ticks <= 0) {
 			ticks = 1;
@@ -162,11 +160,13 @@ static int npcx_itim_start_evt_tmr_by_tick(int32_t ticks)
 		if (unlikely(next_cycs <= curr)) {
 			cyc_evt_timeout = 1;
 		} else {
+			uint32_t dticks;
+
 			dcycles = next_cycs - curr;
-			cyc_evt_timeout =
-				CLAMP((dcycles / SYS_CYC_PER_EVT_CYC), 1, NPCX_ITIM32_MAX_CNT);
+			dticks = DIV_ROUND_UP(dcycles * EVT_CYCLES_PER_SEC,
+					      sys_clock_hw_cycles_per_sec());
+			cyc_evt_timeout = CLAMP(dticks, 1, NPCX_ITIM32_MAX_CNT);
 		}
-
 	}
 	LOG_DBG("ticks %x, cyc_evt_timeout %x", ticks, cyc_evt_timeout);
 
@@ -178,7 +178,6 @@ static int npcx_itim_start_evt_tmr_by_tick(int32_t ticks)
 	/* Upload counter of event timer */
 	evt_tmr->ITCNT32 = MAX(cyc_evt_timeout - 1, 1);
 
-	k_spin_unlock(&lock, key);
 	/* Enable event timer and start ticking */
 	return npcx_itim_evt_enable();
 }
@@ -235,13 +234,13 @@ static uint32_t npcx_itim_evt_elapsed_cyc32(void)
 {
 	uint32_t cnt1 = npcx_itim_get_evt_cyc32();
 	uint8_t  sys_cts = evt_tmr->ITCTS32;
-	uint16_t cnt2 = npcx_itim_get_evt_cyc32();
+	uint32_t cnt2 = npcx_itim_get_evt_cyc32();
 
 	/* Event has been triggered but timer ISR doesn't handle it */
 	if (IS_BIT_SET(sys_cts, NPCX_ITCTSXX_TO_STS) || (cnt2 > cnt1)) {
 		cnt2 = cyc_evt_timeout;
 	} else {
-		cnt2 = cyc_evt_timeout - cnt2;
+		cnt2 = cyc_evt_timeout - cnt2 - 1;
 	}
 
 	/* Return elapsed cycles of 32-bit counter of event timer  */
@@ -261,7 +260,10 @@ void sys_clock_set_timeout(int32_t ticks, bool idle)
 
 	LOG_DBG("timeout is %d", ticks);
 	/* Start a event timer in ticks */
+
+	k_spinlock_key_t key = k_spin_lock(&lock);
 	npcx_itim_start_evt_tmr_by_tick(ticks);
+	k_spin_unlock(&lock, key);
 }
 
 uint32_t sys_clock_elapsed(void)
@@ -273,7 +275,7 @@ uint32_t sys_clock_elapsed(void)
 
 	k_spinlock_key_t key = k_spin_lock(&lock);
 	uint64_t delta_cycle = npcx_itim_get_sys_cyc64() - cyc_sys_announced;
-	uint32_t delta_ticks = (uint32_t)delta_cycle / SYS_CYCLES_PER_TICK;
+	uint32_t delta_ticks = (uint32_t)(delta_cycle / SYS_CYCLES_PER_TICK);
 
 	last_elapsed = delta_ticks;
 	k_spin_unlock(&lock, key);