diff --git a/include/sys/timeutil.h b/include/sys/timeutil.h
index 92acecddc89..ddc437b590e 100644
--- a/include/sys/timeutil.h
+++ b/include/sys/timeutil.h
@@ -32,7 +32,7 @@ extern "C" {
  *
  * @see http://man7.org/linux/man-pages/man3/timegm.3.html
  */
-time_t timeutil_timegm(struct tm *tm);
+time_t timeutil_timegm(const struct tm *tm);
 
 #ifdef __cplusplus
 }
diff --git a/lib/os/timeutil.c b/lib/os/timeutil.c
index 745f158b64a..40a72fc0472 100644
--- a/lib/os/timeutil.c
+++ b/lib/os/timeutil.c
@@ -48,7 +48,7 @@ static s64_t time_days_from_civil(s64_t y,
  * @return the signed number of seconds between 1970-01-01T00:00:00
  * and the specified time ignoring leap seconds and DST offsets.
  */
-time_t timeutil_timegm(struct tm *tm)
+time_t timeutil_timegm(const struct tm *tm)
 {
 	s64_t y = 1900 + (s64_t)tm->tm_year;
 	unsigned int m = tm->tm_mon + 1;