diff --git a/app/controllers/stats_controller.rb b/app/controllers/stats_controller.rb
index 1202df9c8..3032e67c7 100644
--- a/app/controllers/stats_controller.rb
+++ b/app/controllers/stats_controller.rb
@@ -31,7 +31,7 @@ class StatsController < ApplicationController
     @procedures_in_the_last_4_months = last_four_months_serie(procedures, :published_at)
 
     @dossiers_cumulative = stat.dossiers_cumulative
-    @dossiers_in_the_last_4_months = stat.dossiers_in_the_last_4_months
+    @dossiers_in_the_last_4_months = format_keys_as_months(stat.dossiers_in_the_last_4_months)
   end
 
   def download
@@ -136,11 +136,18 @@ class StatsController < ApplicationController
     end
   end
 
+  def format_keys_as_months(series)
+    series.transform_keys do |k|
+      date = k.is_a?(Date) ? k : (Date.parse(k) rescue k)
+      l(date, format: "%B %Y")
+    end
+  end
+
   def last_four_months_serie(association, date_attribute)
-    association
+    series = association
       .group_by_month(date_attribute, last: 4, current: super_admin_signed_in?)
       .count
-      .transform_keys { |date| l(date, format: "%B %Y") }
+    format_keys_as_months(series)
   end
 
   def cumulative_month_serie(association, date_attribute)
diff --git a/app/models/stat.rb b/app/models/stat.rb
index ce0109c5b..097c0c592 100644
--- a/app/models/stat.rb
+++ b/app/models/stat.rb
@@ -80,11 +80,7 @@ class Stat < ApplicationRecord
         association.group_by_month(date_attribute, last: 4, current: false).count
       end
 
-      month_serie(sum_hashes(*timeseries))
-    end
-
-    def month_serie(date_serie)
-      date_serie.keys.sort.each_with_object({}) { |date, h| h[I18n.l(date, format: "%B %Y")] = date_serie[date] }
+      sum_hashes(*timeseries).sort.to_h
     end
 
     def cumulative_month_serie(associations_with_date_attribute)
diff --git a/spec/models/stat_spec.rb b/spec/models/stat_spec.rb
index e9fc2edc2..41138d59a 100644
--- a/spec/models/stat_spec.rb
+++ b/spec/models/stat_spec.rb
@@ -57,24 +57,30 @@ describe Stat do
         create(:dossier, state: :en_construction, depose_at: i.months.ago)
         create(:deleted_dossier, dossier_id: i + 100, state: :en_construction, deleted_at: i.month.ago)
       end
-      rs = Stat.send(:cumulative_month_serie, [
-        [Dossier.state_not_brouillon, :depose_at],
-        [DeletedDossier.where.not(state: :brouillon), :deleted_at]
+      s = Stat.new({
+        dossiers_cumulative:
+                        Stat.send(:cumulative_month_serie, [
+                          [Dossier.state_not_brouillon, :depose_at],
+                          [DeletedDossier.where.not(state: :brouillon), :deleted_at]
+                        ])
+      })
+      s.save!
+      s.reload
+      # Use `Hash#to_a` to also test the key ordering
+      expect(s.dossiers_cumulative.to_a).to eq([
+        [formatted_n_months_ago(12), 2],
+        [formatted_n_months_ago(11), 4],
+        [formatted_n_months_ago(10), 6],
+        [formatted_n_months_ago(9), 8],
+        [formatted_n_months_ago(8), 10],
+        [formatted_n_months_ago(7), 12],
+        [formatted_n_months_ago(6), 14],
+        [formatted_n_months_ago(5), 16],
+        [formatted_n_months_ago(4), 18],
+        [formatted_n_months_ago(3), 20],
+        [formatted_n_months_ago(2), 22],
+        [formatted_n_months_ago(1), 24]
       ])
-      expect(rs).to eq({
-        12 => 2,
-        11 => 4,
-        10 => 6,
-        9 => 8,
-        8 => 10,
-        7 => 12,
-        6 => 14,
-        5 => 16,
-        4 => 18,
-        3 => 20,
-        2 => 22,
-        1 => 24
-      }.transform_keys { |i| i.months.ago.beginning_of_month.to_date })
     end
   end
 
@@ -85,16 +91,22 @@ describe Stat do
           create(:dossier, state: :en_construction, depose_at: i.months.ago)
           create(:deleted_dossier, dossier_id: i + 100, state: :en_construction, deleted_at: i.month.ago)
         end
-        rs = Stat.send(:last_four_months_serie, [
-          [Dossier.state_not_brouillon, :depose_at],
-          [DeletedDossier.where.not(state: :brouillon), :deleted_at]
-        ])
-        expect(rs).to eq({
-          "juillet 2021" => 2,
-          "août 2021" => 2,
-          "septembre 2021" => 2,
-          "octobre 2021" => 2
+        s = Stat.new({
+          dossiers_in_the_last_4_months:
+                                   Stat.send(:last_four_months_serie, [
+                                     [Dossier.state_not_brouillon, :depose_at],
+                                     [DeletedDossier.where.not(state: :brouillon), :deleted_at]
+                                   ])
         })
+        s.save!
+        s.reload
+        # Use `Hash#to_a` to also test the key ordering
+        expect(s.dossiers_in_the_last_4_months.to_a).to eq([
+          ['2021-07-01', 2],
+          ['2021-08-01', 2],
+          ['2021-09-01', 2],
+          ['2021-10-01', 2]
+        ])
       end
     end
   end
@@ -104,4 +116,8 @@ describe Stat do
       expect(Stat.send(:sum_hashes, *[{ a: 1, b: 2, d: 5 }, { a: 2, b: 3, c: 5 }])).to eq({ a: 3, b: 5, c: 5, d: 5 })
     end
   end
+
+  def formatted_n_months_ago(i)
+    i.months.ago.beginning_of_month.to_date.to_s
+  end
 end