diff --git a/CHANGELOG b/CHANGELOG
index 4b4bc25..b777098 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,3 +1,4 @@
+* Timedelta uses 30 days for month
 * Manage context model of ir.action.act_window
 * Add Note
 
diff --git a/src/common.js b/src/common.js
index e42018b..7756567 100644
--- a/src/common.js
+++ b/src/common.js
@@ -275,7 +275,7 @@
     Sao.common.timedelta.DEFAULT_CONVERTER.w =
         Sao.common.timedelta.DEFAULT_CONVERTER.d * 7;
     Sao.common.timedelta.DEFAULT_CONVERTER.M =
-        Sao.common.timedelta.DEFAULT_CONVERTER.w * 4;
+        Sao.common.timedelta.DEFAULT_CONVERTER.d * 30;
     Sao.common.timedelta.DEFAULT_CONVERTER.Y =
         Sao.common.timedelta.DEFAULT_CONVERTER.d * 365;
     Sao.common.timedelta._get_separator = function() {
diff --git a/tests/sao.js b/tests/sao.js
index 9633581..b846389 100644
--- a/tests/sao.js
+++ b/tests/sao.js
@@ -843,9 +843,13 @@
     var timedelta_tests = [
         [null, ''],
         [Sao.TimeDelta(3, 0, 0, 30, 5), '3d 05:30'],
-        [Sao.TimeDelta(0, 0, 0, 0, 0, 48), '12M'],
-        [Sao.TimeDelta(0, 0, 0, 0, 0, 50), '12M 2w'],
+        [Sao.TimeDelta(0, 0, 0, 0, 0, 48), '11M 6d'],
+        [Sao.TimeDelta(0, 0, 0, 0, 0, 50), '11M 2w 6d'],
+        [Sao.TimeDelta(0, 0, 0, 0, 0, 52), '12M 4d'],
+        [Sao.TimeDelta(360), '12M'],
+        [Sao.TimeDelta(364), '12M 4d'],
         [Sao.TimeDelta(365), '1Y'],
+        [Sao.TimeDelta(366), '1Y 1d'],
         [Sao.TimeDelta(0, 10, 0, 5, 2), '02:05:10'],
         [Sao.TimeDelta(0, 0, 42, 15), '00:15:00.042000'],
         [Sao.TimeDelta(1, 0, 42), '1d .042000'],