diff --git a/chrono/timespan.h b/chrono/timespan.h
index ab10e4a..6e799e5 100644
--- a/chrono/timespan.h
+++ b/chrono/timespan.h
@@ -4,6 +4,7 @@
 #include "../conversion/types.h"
 #include "../global.h"
 
+#include <functional>
 #include <limits>
 #include <string>