diff --git a/Modules/datetimemodule.c b/Modules/datetimemodule.c
index 8207ffd..a67c35d 100644
--- a/Modules/datetimemodule.c
+++ b/Modules/datetimemodule.c
@@ -13,7 +13,9 @@
 /* Differentiate between building the core module and building extension
  * modules.
  */
+#ifndef Py_BUILD_CORE
 #define Py_BUILD_CORE
+#endif
 #include "datetime.h"
 #undef Py_BUILD_CORE