-std::ostream& operator<< (std::ostream& s, ContentTime t);
-std::ostream& operator<< (std::ostream& s, DCPTime t);
-std::ostream& operator<< (std::ostream& s, DCPTimePeriod p);
+std::string to_string (ContentTime t);
+std::string to_string (DCPTime t);
+std::string to_string (DCPTimePeriod p);