diff --git a/src/Duration.cpp b/src/Duration.cpp
index 3ce20f57c..f4422e0da 100644
--- a/src/Duration.cpp
+++ b/src/Duration.cpp
@@ -237,7 +237,7 @@ bool Duration::negative () const
 }
 
 ////////////////////////////////////////////////////////////////////////////////
-bool Duration::valid (const std::string& input) const
+bool Duration::valid (const std::string& input)
 {
   std::string lower_input = lowerCase (input);
 
diff --git a/src/Duration.h b/src/Duration.h
index 0897fecf9..b43e5b9eb 100644
--- a/src/Duration.h
+++ b/src/Duration.h
@@ -50,7 +50,7 @@ public:
   std::string formatCompact () const;
 
   bool negative () const;
-  bool valid (const std::string&) const;
+  static bool valid (const std::string&);
   void parse (const std::string&);
 
 private: