diff --git a/include/libdivecomputer/datetime.h b/include/libdivecomputer/datetime.h
index 748a736..d8f596b 100644
--- a/include/libdivecomputer/datetime.h
+++ b/include/libdivecomputer/datetime.h
@@ -53,7 +53,7 @@ dc_datetime_gmtime (dc_datetime_t *result,
                     dc_ticks_t ticks);
 
 dc_ticks_t
-dc_datetime_mktime (dc_datetime_t *dt);
+dc_datetime_mktime (const dc_datetime_t *dt);
 
 #ifdef __cplusplus
 }
diff --git a/src/datetime.c b/src/datetime.c
index 41aaeab..9d07f8c 100644
--- a/src/datetime.c
+++ b/src/datetime.c
@@ -112,7 +112,7 @@ dc_datetime_gmtime (dc_datetime_t *result,
 }
 
 dc_ticks_t
-dc_datetime_mktime (dc_datetime_t *dt)
+dc_datetime_mktime (const dc_datetime_t *dt)
 {
 	if (dt == NULL)
 		return -1;