--- a/python/mozbuild/mozbuild/backend/base.py
+++ b/python/mozbuild/mozbuild/backend/base.py
@@ -169,19 +169,20 @@ class BuildBackend(LoggingMixin):
                 pass
 
         # Write out the list of backend files generated, if it changed.
         if self._deleted_count or self._created_count or \
                 not os.path.exists(list_file):
             with self._write_file(list_file) as fh:
                 fh.write('\n'.join(sorted(self._backend_output_files)))
         else:
-            # Always update its mtime.
-            with open(list_file, 'a'):
-                os.utime(list_file, None)
+            # Always update its mtime if we're not in dry-run mode.
+            if not self.dry_run:
+                with open(list_file, 'a'):
+                    os.utime(list_file, None)
 
         # Write out the list of input files for the backend
         with self._write_file('%s.in' % list_file) as fh:
             fh.write('\n'.join(sorted(
                 mozpath.normsep(f) for f in self.backend_input_files)))
 
     @abstractmethod
     def consume_object(self, obj):