diff --git a/src/Daterange.cpp b/src/Daterange.cpp
index 6bc763c6..15c001fd 100644
--- a/src/Daterange.cpp
+++ b/src/Daterange.cpp
@@ -28,7 +28,7 @@
 #include <Daterange.h>
 
 ////////////////////////////////////////////////////////////////////////////////
-Daterange::Daterange (const Datetime start, const Datetime end)
+Daterange::Daterange (const Datetime& start, const Datetime& end)
 {
   _start = start;
   _end = end;
@@ -41,7 +41,7 @@ Datetime Daterange::start () const
 }
 
 ////////////////////////////////////////////////////////////////////////////////
-void Daterange::start (Datetime value)
+void Daterange::start (const Datetime& value)
 {
   _start = value;
 }
@@ -53,7 +53,7 @@ Datetime Daterange::end () const
 }
 
 ////////////////////////////////////////////////////////////////////////////////
-void Daterange::end (Datetime value)
+void Daterange::end (const Datetime& value)
 {
   _end = value;
 }
@@ -91,7 +91,7 @@ bool Daterange::isEnded () const
 // other                  |...                             true
 // other                      |...                         true
 //
-bool Daterange::overlap (const Daterange other) const
+bool Daterange::overlap (const Daterange& other) const
 {
   if (! isStarted () ||
       ! other.isStarted ())
diff --git a/src/Daterange.h b/src/Daterange.h
index b0f0830b..896e3e9c 100644
--- a/src/Daterange.h
+++ b/src/Daterange.h
@@ -33,17 +33,17 @@ class Daterange
 {
 public:
   Daterange () = default;
-  Daterange (const Datetime, const Datetime);
+  Daterange (const Datetime&, const Datetime&);
 
   Datetime start () const;
-  void start (Datetime);
+  void start (const Datetime&);
 
   Datetime end () const;
-  void end (Datetime);
+  void end (const Datetime&);
 
   bool isStarted () const;
   bool isEnded () const;
-  bool overlap (const Daterange) const;
+  bool overlap (const Daterange&) const;
 
 private:
   Datetime               _start {0};