diff --git a/sys/include/rtc_utils.h b/sys/include/rtc_utils.h
index cd9d197bd0cb..88a983502ab7 100644
--- a/sys/include/rtc_utils.h
+++ b/sys/include/rtc_utils.h
@@ -69,7 +69,7 @@ int rtc_tm_compare(const struct tm *a, const struct tm *b);
  *
  * @return            elapsed seconds since `RIOT_EPOCH`
  */
-uint32_t rtc_mktime(struct tm *t);
+uint32_t rtc_mktime(const struct tm *t);
 
 /**
  * @brief Converts an RTC timestamp into a  time struct.
diff --git a/sys/rtc_utils/rtc_utils.c b/sys/rtc_utils/rtc_utils.c
index 6c1c792179c3..4c2c10a66144 100644
--- a/sys/rtc_utils/rtc_utils.c
+++ b/sys/rtc_utils/rtc_utils.c
@@ -177,7 +177,7 @@ void rtc_tm_normalize(struct tm *t)
 #endif
 }
 
-uint32_t rtc_mktime(struct tm *t)
+uint32_t rtc_mktime(const struct tm *t)
 {
     unsigned year = t->tm_year + 1900;
     uint32_t time = t->tm_sec