diff --git a/drivers/rtc/rtc-cmos.c b/drivers/rtc/rtc-cmos.c
index e3fe83a..29cf145 100644
--- a/drivers/rtc/rtc-cmos.c
+++ b/drivers/rtc/rtc-cmos.c
@@ -120,7 +120,8 @@
 	t->time.tm_hour = CMOS_READ(RTC_HOURS_ALARM);
 
 	if (cmos->day_alrm) {
-		t->time.tm_mday = CMOS_READ(cmos->day_alrm);
+		/* ignore upper bits on readback per ACPI spec */
+		t->time.tm_mday = CMOS_READ(cmos->day_alrm) & 0x3f;
 		if (!t->time.tm_mday)
 			t->time.tm_mday = -1;