diff --git a/hw/mc146818rtc.c b/hw/mc146818rtc.c
index ec7c4ecdb6..a1b0e31ee5 100644
--- a/hw/mc146818rtc.c
+++ b/hw/mc146818rtc.c
@@ -247,7 +247,15 @@ static void cmos_ioport_write(void *opaque, uint32_t addr, uint32_t data)
                     rtc_set_time(s);
                 }
             }
-            s->cmos_data[RTC_REG_B] = data;
+            if (((s->cmos_data[RTC_REG_B] ^ data) & (REG_B_DM | REG_B_24H)) &&
+                !(data & REG_B_SET)) {
+                /* If the time format has changed and not in set mode,
+                   update the registers immediately. */
+                s->cmos_data[RTC_REG_B] = data;
+                rtc_copy_date(s);
+            } else {
+                s->cmos_data[RTC_REG_B] = data;
+            }
             rtc_timer_update(s, qemu_get_clock(rtc_clock));
             break;
         case RTC_REG_C: