diff --git a/lib/core-net/private-lib-core-net.h b/lib/core-net/private-lib-core-net.h
index a688e03b5..c28a29642 100644
--- a/lib/core-net/private-lib-core-net.h
+++ b/lib/core-net/private-lib-core-net.h
@@ -1447,6 +1447,9 @@ lws_plat_mbedtls_net_send(void *ctx, const uint8_t *buf, size_t len);
 int
 lws_plat_mbedtls_net_recv(void *ctx, unsigned char *buf, size_t len);
 
+lws_usec_t
+lws_sul_nonmonotonic_adjust(struct lws_context *ctx, int64_t step_us);
+
 
 enum {
 	LW5CHS_RET_RET0,
diff --git a/lib/core-net/sorted-usec-list.c b/lib/core-net/sorted-usec-list.c
index 04de59168..895865c56 100644
--- a/lib/core-net/sorted-usec-list.c
+++ b/lib/core-net/sorted-usec-list.c
@@ -103,7 +103,7 @@ __lws_sul_service_ripe(lws_dll2_owner_t *own, int own_len, lws_usec_t usnow)
 		lws_system_do_attach(pt);
 
 	/* must be at least 1 */
-	assert(own_len);
+	assert(own_len > 0);
 
 	/*
 	 * Of the own_len sul owning lists, the earliest next sul could be on
@@ -154,6 +154,72 @@ __lws_sul_service_ripe(lws_dll2_owner_t *own, int own_len, lws_usec_t usnow)
 	return 0;
 }
 
+/*
+ * Normally we use the OS monotonic time, which does not step when the
+ * gettimeofday() time is adjusted after, eg, ntpclient.  But on some OSes,
+ * high resolution monotonic time doesn't exist; sul time is computed from and
+ * compared against gettimeofday() time and breaks when that steps.
+ *
+ * For those cases, this allows us to retrospectively adjust existing suls on
+ * all owning lists by the step amount, at the same time we adjust the
+ * nonmonotonic clock.  Then nothing breaks so long as we do this when the
+ * gettimeofday() clock is stepped.
+ *
+ * Linux and so on offer Posix MONOTONIC, which lws uses.  FreeRTOS doesn't
+ * have a high-resolution monotonic clock and has to use gettimeofday(), which
+ * requires this adjustment when it is stepped.
+ */
+
+lws_usec_t
+lws_sul_nonmonotonic_adjust(struct lws_context *ctx, int64_t step_us)
+{
+	struct lws_context_per_thread *pt = &ctx->pt[0];
+	int n, m;
+
+	/*
+	 * for each pt
+	 */
+
+	for (m = 0; m < ctx->count_threads; m++) {
+
+		/*
+		 * For each owning list...
+		 */
+
+		lws_pt_lock(pt, __func__);
+
+		for (n = 0; n < LWS_COUNT_PT_SUL_OWNERS; n++) {
+
+			if (!pt->pt_sul_owner[n].count)
+				continue;
+
+			/* ... and for every existing sul on a list... */
+
+			lws_start_foreach_dll(struct lws_dll2 *, p,
+					      lws_dll2_get_head(
+							&pt->pt_sul_owner[n])) {
+				lws_sorted_usec_list_t *sul = lws_container_of(
+					       p, lws_sorted_usec_list_t, list);
+
+				/*
+				 * ... retrospectively step its ripe time by the
+				 * step we will adjust the gettimeofday() clock
+				 * with
+				 */
+
+				sul->us += step_us;
+
+			} lws_end_foreach_dll(p);
+		}
+
+		lws_pt_unlock(pt);
+
+		pt++;
+	}
+
+	return 0;
+}
+
 /*
  * Earliest wakeable event on any pt
  */
diff --git a/lib/system/ntpclient/ntpclient.c b/lib/system/ntpclient/ntpclient.c
index 012c04196..04e2bad6c 100644
--- a/lib/system/ntpclient/ntpclient.c
+++ b/lib/system/ntpclient/ntpclient.c
@@ -127,6 +127,8 @@ callback_ntpc(struct lws *wsi, enum lws_callback_reasons reason, void *user,
 			lws_protocol_vh_priv_get(lws_get_vhost(wsi),
 						 lws_get_protocol(wsi));
 	uint8_t pkt[LWS_PRE + 48];
+	struct timeval t1;
+	int64_t delta_us;
 	uint64_t ns;
 
 	switch (reason) {
@@ -224,16 +226,28 @@ do_close:
 		ns = lws_ser_ru32be(((uint8_t *)in) + 40) - 2208988800;
 		ns = (ns * 1000000000) + lws_ser_ru32be(((uint8_t *)in) + 44);
 
-		lwsl_notice("%s: Unix time: %llu\n", __func__,
-				(unsigned long long)ns / 1000000000);
+		/*
+		 * Compute the step
+		 */
 
-#if defined(LWS_ESP_PLATFORM)
+		gettimeofday(&t1, NULL);
+
+		delta_us = (ns / 1000) -
+				((t1.tv_sec * LWS_US_PER_SEC) + t1.tv_usec);
+
+		lwsl_notice("%s: Unix time: %llu, step: %lldus\n", __func__,
+				(unsigned long long)ns / 1000000000,
+				delta_us);
+
+#if defined(LWS_PLAT_FREERTOS)
 		{
 			struct timeval t;
 
 			t.tv_sec = (unsigned long long)ns / 1000000000;
 			t.tv_usec = (ns % 1000000000) / 1000;
 
+			lws_sul_nonmonotonic_adjust(wsi->context, delta_us);
+
 			settimeofday(&t, NULL);
 		}
 #endif