diff --git a/utility.c b/utility.c
index 49ad52c..64e81a8 100644
--- a/utility.c
+++ b/utility.c
@@ -70,7 +70,7 @@
 
 
 long int
-get_time_diff(char *time1, char *time2)
+get_time_diff(const char *time1, const char *time2)
 {
 	long int diff_secs;