diff --git a/drivers/rtc/rtc-cmos.c b/drivers/rtc/rtc-cmos.c
index b48517021ee6..dcdc142a3441 100644
--- a/drivers/rtc/rtc-cmos.c
+++ b/drivers/rtc/rtc-cmos.c
@@ -198,9 +198,8 @@ static int cmos_set_alarm(struct device *dev, struct rtc_wkalrm *t)
 
 	/* Writing 0xff means "don't care" or "match all".  */
 
-	mon = t->time.tm_mon;
-	mon = (mon < 12) ? BIN2BCD(mon) : 0xff;
-	mon++;
+	mon = t->time.tm_mon + 1;
+	mon = (mon <= 12) ? BIN2BCD(mon) : 0xff;
 
 	mday = t->time.tm_mday;
 	mday = (mday >= 1 && mday <= 31) ? BIN2BCD(mday) : 0xff;