diff --git a/sys_macosx.c b/sys_macosx.c
index c1592dd..fb2a70e 100644
--- a/sys_macosx.c
+++ b/sys_macosx.c
@@ -248,7 +248,7 @@ apply_step_offset(double offset)
     return 0;
   }
 
-  UTI_AddDoubleToTimeval(&T0, offset, &T1);
+  UTI_AddDoubleToTimeval(&T0, -offset, &T1);
   T0 = T1;
 
   start_adjust();