diff --git a/date.h b/date.h
index e6e9dd2..536add0 100644
--- a/date.h
+++ b/date.h
@@ -40,6 +40,7 @@
 #include <ratio>
 #include <type_traits>
 #include <sstream>
+#include <cctype>
 
 namespace date
 {