diff --git a/builder.py b/builder.py
index e3c9df7..3dfe66f 100644
--- a/builder.py
+++ b/builder.py
@@ -236,7 +236,7 @@ class BuildJob:
         st1 = os.fstat(f.fileno())
         st2 = _try_stat(self.tmpname2)
         if (after_t and 
-            (not before_t or before_t.st_ctime != after_t.st_ctime) and
+            (not before_t or before_t.st_mtime != after_t.st_mtime) and
             not stat.S_ISDIR(after_t.st_mode)):
             err('%s modified %s directly!\n' % (self.argv[2], t))
             err('...you should update $3 (a temp file) or stdout, not $1.\n')