diff --git a/drivers/rtt_rtc/rtt_rtc.c b/drivers/rtt_rtc/rtt_rtc.c
index 0de3fe296c..2e3bed2dc3 100644
--- a/drivers/rtt_rtc/rtt_rtc.c
+++ b/drivers/rtt_rtc/rtt_rtc.c
@@ -182,6 +182,8 @@ int rtc_set_alarm(struct tm *time, rtc_alarm_cb_t cb, void *arg)
 
     uint32_t now = rtt_get_counter();
 
+    rtc_tm_normalize(time);
+
     alarm_time   = rtc_mktime(time);
     alarm_cb_arg = arg;
     alarm_cb     = cb;