diff --git a/src/helper/time_support.c b/src/helper/time_support.c
index c5978b423..780eb9694 100644
--- a/src/helper/time_support.c
+++ b/src/helper/time_support.c
@@ -78,14 +78,14 @@ int duration_measure(struct duration *duration)
 	return retval;
 }
 
-float duration_elapsed(struct duration *duration)
+float duration_elapsed(const struct duration *duration)
 {
 	float t = duration->elapsed.tv_sec;
 	t += (float)duration->elapsed.tv_usec / 1000000.0;
 	return t;
 }
 
-float duration_kbps(struct duration *duration, size_t count)
+float duration_kbps(const struct duration *duration, size_t count)
 {
 	return count / (1024.0 * duration_elapsed(duration));
 }
diff --git a/src/helper/time_support.h b/src/helper/time_support.h
index ab839826b..5a95e7eee 100644
--- a/src/helper/time_support.h
+++ b/src/helper/time_support.h
@@ -55,8 +55,8 @@ int duration_start(struct duration *duration);
 int duration_measure(struct duration *duration);
 
 /** @returns Elapsed time in seconds. */
-float duration_elapsed(struct duration *duration);
+float duration_elapsed(const struct duration *duration);
 /** @returns KB/sec for the elapsed @a duration and @a count bytes. */
-float duration_kbps(struct duration *duration, size_t count);
+float duration_kbps(const struct duration *duration, size_t count);
 
 #endif	/* TIME_SUPPORT_H */