diff --git a/utility.h b/utility.h
index 75bf5ab..6015ecc 100644
--- a/utility.h
+++ b/utility.h
@@ -5,6 +5,6 @@
 char * getGmTimeStamp(void);
 long int get_current_time_sec(void);
 void get_current_timestamp_micro(char * microSec);
-long int get_time_diff(char *time1, char *time2);
+long int get_time_diff(const char *time1, const char *time2);
 
 #endif