diff --git a/traffxml/traff_model.cpp b/traffxml/traff_model.cpp
index ec981e489..e7cd9a515 100644
--- a/traffxml/traff_model.cpp
+++ b/traffxml/traff_model.cpp
@@ -145,16 +145,16 @@ bool IsoTime::IsPast()
   return t_tm < t_now;
 }
 
-bool operator< (IsoTime lhs, IsoTime rhs)
+bool IsoTime::operator< (IsoTime & rhs)
 {
-  std::time_t t_lhs = std::mktime(&lhs.m_tm);
+  std::time_t t_lhs = std::mktime(&m_tm);
   std::time_t t_rhs = std::mktime(&rhs.m_tm);
   return t_lhs < t_rhs;
 }
 
-bool operator> (IsoTime lhs, IsoTime rhs)
+bool IsoTime::operator> (IsoTime & rhs)
 {
-  std::time_t t_lhs = std::mktime(&lhs.m_tm);
+  std::time_t t_lhs = std::mktime(&m_tm);
   std::time_t t_rhs = std::mktime(&rhs.m_tm);
   return t_lhs > t_rhs;
 }
diff --git a/traffxml/traff_model.hpp b/traffxml/traff_model.hpp
index 0ab998e94..b73c28ce5 100644
--- a/traffxml/traff_model.hpp
+++ b/traffxml/traff_model.hpp
@@ -74,8 +74,8 @@ public:
    */
   bool IsPast();
 
-  friend bool operator< (IsoTime lhs, IsoTime rhs);
-  friend bool operator> (IsoTime lhs, IsoTime rhs);
+  bool operator< (IsoTime & rhs);
+  bool operator> (IsoTime & rhs);
 private:
   friend std::string DebugPrint(IsoTime time);