1 files changed, 2 insertions, 1 deletions
diff --git a/setup.py b/setup.py
index ef2faf613be..b220f5279ca 100644
--- a/setup.py
+++ b/setup.py
@@ -853,7 +853,8 @@ class PyBuildExt(build_ext):
         # libm is needed by delta_new() that uses round() and by accum() that
         # uses modf().
         self.add(Extension('_datetime', ['_datetimemodule.c'],
-                           libraries=['m']))
+                           libraries=['m'],
+                           extra_compile_args=['-DPy_BUILD_CORE_MODULE']))
         # zoneinfo module
         self.add(Extension('_zoneinfo', ['_zoneinfo.c'])),
         # random number generator implemented in C