diff --git a/opam b/opam
index 17f8645f..2dbec1b2 100644
--- a/opam
+++ b/opam
@@ -30,6 +30,7 @@ depends: [
   "yojson" {>= "1.6.0"}
   "resource-pooling" {>= "1.0" & < "2.0"}
   "cohttp-lwt-unix"
+  "js_of_ocaml" {>= "5.5.0"}
   "re" {>= "1.7.2"}
 ]
 depexts: [
diff --git a/src/os_date.eliom b/src/os_date.eliom
index 00b2a7bd..6b0cc23a 100644
--- a/src/os_date.eliom
+++ b/src/os_date.eliom
@@ -141,14 +141,20 @@ let to_utc ?(timezone = user_tz ()) d =
 
 let%client to_local d =
   let d = CalendarLib.Calendar.to_unixfloat d in
-  let o = (new%js Js.date_fromTimeValue (d *. 1000.))##getTimezoneOffset in
+  let o =
+    (new%js Js.date_fromTimeValue (Js.float (d *. 1000.)))##getTimezoneOffset
+  in
   CalendarLib.Calendar.from_unixfloat (d -. (float o *. 60.))
 
 let%client to_utc d =
   let d = CalendarLib.Calendar.to_unixfloat d in
-  let o = (new%js Js.date_fromTimeValue (d *. 1000.))##getTimezoneOffset in
+  let o =
+    (new%js Js.date_fromTimeValue (Js.float (d *. 1000.)))##getTimezoneOffset
+  in
   let d' = d +. (float o *. 60.) in
-  let o' = (new%js Js.date_fromTimeValue (d' *. 1000.))##getTimezoneOffset in
+  let o' =
+    (new%js Js.date_fromTimeValue (Js.float (d' *. 1000.)))##getTimezoneOffset
+  in
   CalendarLib.Calendar.from_unixfloat
     (if o = o'
      then d' (* We guessed the DST status right *)