diff --git a/toml/datetime.hpp b/toml/datetime.hpp
index dc52396..371f93b 100644
--- a/toml/datetime.hpp
+++ b/toml/datetime.hpp
@@ -20,7 +20,7 @@ namespace toml
 namespace detail
 {
 // TODO: find more sophisticated way to handle this
-#if _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _BSD_SOURCE || _SVID_SOURCE || _POSIX_SOURCE
+#if _POSIX_C_SOURCE >= 1 || defined(_XOPEN_SOURCE) || defined(_BSD_SOURCE) || defined(_SVID_SOURCE) || defined(_POSIX_SOURCE)
 inline std::tm localtime_s(const std::time_t* src)
 {
     std::tm dst;