diff --git a/drivers/rtc/interface.c b/drivers/rtc/interface.c
index eb4c883..38d1dc7 100644
--- a/drivers/rtc/interface.c
+++ b/drivers/rtc/interface.c
@@ -227,11 +227,11 @@
 		alarm->time.tm_hour = now.tm_hour;
 
 	/* For simplicity, only support date rollover for now */
-	if (alarm->time.tm_mday == -1) {
+	if (alarm->time.tm_mday < 1 || alarm->time.tm_mday > 31) {
 		alarm->time.tm_mday = now.tm_mday;
 		missing = day;
 	}
-	if (alarm->time.tm_mon == -1) {
+	if ((unsigned)alarm->time.tm_mon >= 12) {
 		alarm->time.tm_mon = now.tm_mon;
 		if (missing == none)
 			missing = month;