diff --git a/.nix/catala.nix b/.nix/catala.nix
index 62cedc51..d7a8b96e 100644
--- a/.nix/catala.nix
+++ b/.nix/catala.nix
@@ -7,6 +7,7 @@
 , calendar
 , cmdliner_1_1_0
 , cppo
+, dates_calc
 , fetchFromGitHub
 , js_of_ocaml
 , js_of_ocaml-ppx
@@ -43,6 +44,7 @@ buildDunePackage rec {
     bindlib
     cmdliner_1_1_0
     cppo
+    dates_calc
     js_of_ocaml
     js_of_ocaml-ppx
     menhir
diff --git a/.nix/dates_calc.nix b/.nix/dates_calc.nix
new file mode 100644
index 00000000..d32e7c7b
--- /dev/null
+++ b/.nix/dates_calc.nix
@@ -0,0 +1,16 @@
+{ lib, fetchurl, buildDunePackage }:
+
+buildDunePackage rec {
+  pname = "dates_calc";
+  version = "0.0.4";
+
+  minimumOCamlVersion = "4.11.0";
+
+  duneVersion = "2";
+
+  src = fetchurl
+    {
+      url = "https://github.com/CatalaLang/dates-calc/archive/${version}.tar.gz";
+      sha256 = "sha256-lWhNBK50r3qhY3PXRHZVvqDee6Y4+3vC51ZfjLxGryg=";
+    };
+}
diff --git a/.nix/packages.nix b/.nix/packages.nix
index 09f7865a..9b28fc12 100644
--- a/.nix/packages.nix
+++ b/.nix/packages.nix
@@ -28,4 +28,5 @@ ocamlPackages.overrideScope' (self: super: {
   clerk = self.callPackage ./clerk.nix { };
   ppx_yojson_conv = self.callPackage ./ppx_yojson_conv.nix { };
   ubase = self.callPackage ./ubase.nix { };
+  dates_calc = self.callPackage ./dates_calc.nix { };
 })