diff --git a/cpp/gigasecond/gigasecond.h b/cpp/gigasecond/gigasecond.h
index 48759bd..4b8578e 100644
--- a/cpp/gigasecond/gigasecond.h
+++ b/cpp/gigasecond/gigasecond.h
@@ -10,7 +10,7 @@ namespace gigasecond {
 typedef boost::gregorian::date              date_t;
 typedef boost::gregorian::date_duration     date_duration_t;
 
-static inline date_t advance(date_t date)
+static inline date_t advance(const date_t &date)
 {
     return date + date_duration_t(1000000000 / (60*60*24));
 }