diff --git a/Lib/test/test_os.py b/Lib/test/test_os.py
index 4a21ff7..bc1b0ee 100644
--- a/Lib/test/test_os.py
+++ b/Lib/test/test_os.py
@@ -621,9 +621,12 @@
 
         if not self.support_subsecond(self.fname):
             delta = 1.0
+        elif os.name == 'nt':
+            # On Windows, the usual resolution of time.time() is 15.6 ms.
+            # bpo-30649: Tolerate 50 ms for slow Windows buildbots.
+            delta = 0.050
         else:
-            # On Windows, the usual resolution of time.time() is 15.6 ms
-            delta = 0.020
+            delta = 0.010
         st = os.stat(self.fname)
         msg = ("st_time=%r, current=%r, dt=%r"
                % (st.st_mtime, current, st.st_mtime - current))