diff --git a/build_system/dune b/build_system/dune
index 515077bd..9fe70d05 100644
--- a/build_system/dune
+++ b/build_system/dune
@@ -7,7 +7,13 @@
 (library
  (name clerk_driver)
  (public_name clerk.driver)
- (libraries catala.runtime_ocaml catala.utils ninja_utils cmdliner re ANSITerminal)
+ (libraries
+  catala.runtime_ocaml
+  catala.utils
+  ninja_utils
+  cmdliner
+  re
+  ANSITerminal)
  (modules clerk_driver))
 
 (executable
diff --git a/runtimes/jsoo/dune b/runtimes/jsoo/dune
index 8fa72b53..0cf5f2ef 100644
--- a/runtimes/jsoo/dune
+++ b/runtimes/jsoo/dune
@@ -1,5 +1,7 @@
 (library
  (name runtime_jsoo)
  (public_name catala.runtime_jsoo)
+ (preprocess
+  (pps js_of_ocaml-ppx))
  (libraries catala.runtime_ocaml js_of_ocaml)
  (modules runtime))
diff --git a/runtimes/jsoo/runtime.ml b/runtimes/jsoo/runtime.ml
index 64a63c51..394f5334 100644
--- a/runtimes/jsoo/runtime.ml
+++ b/runtimes/jsoo/runtime.ml
@@ -15,6 +15,7 @@
    the License. *)
 
 open Js_of_ocaml
+module R_ocaml = Runtime_ocaml.Runtime
 
 class type source_position =
   object
@@ -41,10 +42,18 @@ class type event =
 
 class type duration =
   object
-    method years : int Js.prop
-    method months : int Js.prop
-    method days : int Js.prop
+    method years : int Js.readonly_prop
+    method months : int Js.readonly_prop
+    method days : int Js.readonly_prop
   end
 
-(* let duration_of_jsoo d = *)
-(*   Runtime.duration_of_numbers d##.years d##.months d##.days *)
+let duration_of_jsoo d =
+  R_ocaml.duration_of_numbers d##.years d##.months d##.days
+
+let duration_to_jsoo d =
+  let years, months, days = R_ocaml.duration_to_years_months_days d in
+  object%js
+    val years = years
+    val months = months
+    val days = days
+  end
diff --git a/runtimes/jsoo/runtime.mli b/runtimes/jsoo/runtime.mli
index 7bc88e8e..8e829abd 100644
--- a/runtimes/jsoo/runtime.mli
+++ b/runtimes/jsoo/runtime.mli
@@ -44,10 +44,10 @@ class type event =
 
 class type duration =
   object
-    method years : int Js.prop
-    method months : int Js.prop
-    method days : int Js.prop
+    method years : int Js.readonly_prop
+    method months : int Js.readonly_prop
+    method days : int Js.readonly_prop
   end
 
-(* let duration_of_jsoo d = *)
-(*   Runtime.duration_of_numbers d##.years d##.months d##.days *)
+val duration_of_jsoo : duration Js.t -> Runtime_ocaml.Runtime.duration
+val duration_to_jsoo : Runtime_ocaml.Runtime.duration -> duration Js.t