diff --git a/ocaml-calendar.spec b/ocaml-calendar.spec
index 135351a..0ac8d07 100644
--- a/ocaml-calendar.spec
+++ b/ocaml-calendar.spec
@@ -48,6 +48,8 @@ developing applications that use %{name}.
 %autosetup -p1
 
 %build
+sed -i~ '/system date/d' src/dune
+diff -u "$_"~ "$_" && exit 1
 dune_release_pkgs='calendar'
 %ocaml_dune_setup
 %ocaml_dune_build