diff --git a/src/ISO8601.cpp b/src/ISO8601.cpp
index 6b4b16559..d5d9a6dbf 100644
--- a/src/ISO8601.cpp
+++ b/src/ISO8601.cpp
@@ -167,7 +167,10 @@ ISO8601d::operator time_t () const
 //                 | 'R' [n] '/' datetime '/' datetime                # start end
 //                 ;
 //
-bool ISO8601d::parse (const std::string& input, std::string::size_type& start)
+bool ISO8601d::parse (
+  const std::string& input,
+  std::string::size_type& start,
+  const std::string& format /* = "" */)
 {
   auto i = start;
   Nibbler n (input.substr (i));
diff --git a/src/ISO8601.h b/src/ISO8601.h
index 9a6b60e79..9c41c81e1 100644
--- a/src/ISO8601.h
+++ b/src/ISO8601.h
@@ -23,6 +23,7 @@
 // http://www.opensource.org/licenses/mit-license.php
 //
 ////////////////////////////////////////////////////////////////////////////////
+
 #ifndef INCLUDED_ISO8601
 #define INCLUDED_ISO8601
 
@@ -38,7 +39,7 @@ public:
   ISO8601d (const ISO8601d&);                 // Unimplemented
   ISO8601d& operator= (const ISO8601d&);      // Unimplemented
   operator time_t () const;
-  bool parse (const std::string&, std::string::size_type&);
+  bool parse (const std::string&, std::string::size_type&, const std::string& format = "");
   void clear ();
 
 private: