diff --git a/hw/mc146818rtc.c b/hw/mc146818rtc.c
index 98839f2..8f83377 100644
--- a/hw/mc146818rtc.c
+++ b/hw/mc146818rtc.c
@@ -569,7 +569,10 @@ static void rtc_update_time(RTCState *s)
     guest_nsec = get_guest_rtc_ns(s);
     guest_sec = guest_nsec / NSEC_PER_SEC;
     gmtime_r(&guest_sec, &ret);
-    rtc_set_cmos(s, &ret);
+
+    if ((s->cmos_data[RTC_REG_B] & REG_B_SET) != REG_B_SET) {
+        rtc_set_cmos(s, &ret);
+    }
 }
 
 static int update_in_progress(RTCState *s)
diff --git a/tests/rtc-test.c b/tests/rtc-test.c
index 7fdc94a..9b06b63 100644
--- a/tests/rtc-test.c
+++ b/tests/rtc-test.c
@@ -327,6 +327,30 @@ static void fuzz_registers(void)
     }
 }
 
+static void register_b_set_flag(void)
+{
+    /* Enable binary-coded decimal (BCD) mode and SET flag in Register B*/
+    cmos_write(RTC_REG_B, (cmos_read(RTC_REG_B) & ~REG_B_DM) | REG_B_SET);
+    cmos_write(RTC_REG_A, 0x76);
+    cmos_write(RTC_YEAR, 0x11);
+    cmos_write(RTC_CENTURY, 0x20);
+    cmos_write(RTC_MONTH, 0x02);
+    cmos_write(RTC_DAY_OF_MONTH, 0x02);
+    cmos_write(RTC_HOURS, 0x02);
+    cmos_write(RTC_MINUTES, 0x04);
+    cmos_write(RTC_SECONDS, 0x58);
+    cmos_write(RTC_REG_A, 0x26);
+    cmos_write(RTC_REG_B, cmos_read(RTC_REG_B) & ~REG_B_SET);
+
+    g_assert_cmpint(cmos_read(RTC_HOURS), ==, 0x02);
+    g_assert_cmpint(cmos_read(RTC_MINUTES), ==, 0x04);
+    g_assert_cmpint(cmos_read(RTC_SECONDS), >=, 0x58);
+    g_assert_cmpint(cmos_read(RTC_DAY_OF_MONTH), ==, 0x02);
+    g_assert_cmpint(cmos_read(RTC_MONTH), ==, 0x02);
+    g_assert_cmpint(cmos_read(RTC_YEAR), ==, 0x11);
+    g_assert_cmpint(cmos_read(RTC_CENTURY), ==, 0x20);
+}
+
 int main(int argc, char **argv)
 {
     QTestState *s = NULL;
@@ -343,6 +367,7 @@ int main(int argc, char **argv)
     qtest_add_func("/rtc/set-year/20xx", set_year_20xx);
     qtest_add_func("/rtc/set-year/1980", set_year_1980);
     qtest_add_func("/rtc/fuzz-registers", fuzz_registers);
+    qtest_add_func("/rtc/register_b_set_flag", register_b_set_flag);
     ret = g_test_run();
 
     if (s) {