diff --git a/include/toml++/impl/date_time.hpp b/include/toml++/impl/date_time.hpp
index 8957ad33..55b6e29b 100644
--- a/include/toml++/impl/date_time.hpp
+++ b/include/toml++/impl/date_time.hpp
@@ -7,6 +7,7 @@
 #include "forward_declarations.hpp"
 #include "print_to_stream.hpp"
 #include "header_start.hpp"
+#include "std_optional.hpp"
 
 TOML_NAMESPACE_START
 {