diff --git a/app/models/procedure.rb b/app/models/procedure.rb
index 0af87e5a9..abced3d34 100644
--- a/app/models/procedure.rb
+++ b/app/models/procedure.rb
@@ -566,10 +566,14 @@ class Procedure < ApplicationRecord
   end
 
   def usual_traitement_time
+    compute_usual_traitement_time_for_month(Time.zone.now)
+  end
+
+  def compute_usual_traitement_time_for_month(month_date)
     times = Traitement.includes(:dossier)
       .where(dossier: self.dossiers)
       .where.not('dossiers.en_construction_at' => nil, :processed_at => nil)
-      .where(processed_at: 1.month.ago..Time.zone.now)
+      .where(processed_at: (month_date - 1.month)..month_date)
       .pluck('dossiers.en_construction_at', :processed_at)
       .map { |(en_construction_at, processed_at)| processed_at - en_construction_at }
 
diff --git a/spec/models/procedure_spec.rb b/spec/models/procedure_spec.rb
index 1f7327fdb..8e4bebf22 100644
--- a/spec/models/procedure_spec.rb
+++ b/spec/models/procedure_spec.rb
@@ -983,10 +983,6 @@ describe Procedure do
   describe '#usual_traitement_time' do
     let(:procedure) { create(:procedure) }
 
-    def create_dossier(construction_date:, instruction_date:, processed_date:)
-      dossier = create(:dossier, :accepte, procedure: procedure, en_construction_at: construction_date, en_instruction_at: instruction_date, processed_at: processed_date)
-    end
-
     before do
       Timecop.freeze(Time.utc(2019, 6, 1, 12, 0))
 
@@ -1034,6 +1030,36 @@ describe Procedure do
     end
   end
 
+  describe '.compute_usual_traitement_time_for_month' do
+    let(:procedure) { create(:procedure) }
+
+    def create_dossiers(delays_by_month)
+      delays_by_month.each_with_index do |delays, index|
+        delays.each do |delay|
+          processed_date = (index.months + 1.week).ago
+          create_dossier(construction_date: processed_date - delay, instruction_date: processed_date - delay + 12.hours, processed_date: processed_date)
+        end
+      end
+    end
+
+    before do
+      Timecop.freeze(Time.utc(2019, 6, 1, 12, 0))
+
+      create_dossiers(delays_by_month)
+    end
+
+    after { Timecop.return }
+
+    context 'when there are several processed dossiers' do
+      let(:delays_by_month) { [[1.day, 2.days, 2.days, 2.days, 2.days, 3.days, 3.days, 3.days, 3.days, 12.days], [1.month, 2.months, 2.months, 2.months]] }
+
+      it 'computes a time representative of the dossier instruction delay for a specific month' do
+        expect(procedure.compute_usual_traitement_time_for_month(Date.strptime("2019-05", "%Y-%m"))).to be_between(1.month, 2.months)
+        expect(procedure.compute_usual_traitement_time_for_month(Date.strptime("2019-06", "%Y-%m"))).to be_between(3.days, 4.days)
+      end
+    end
+  end
+
   describe '.ensure_a_groupe_instructeur_exists' do
     let!(:procedure) { create(:procedure) }
 
@@ -1082,12 +1108,16 @@ describe Procedure do
     it 'estimates average dossier weight' do
       expect(procedure.reload.average_dossier_weight).to eq 5
     end
+  end
 
-    private
+  private
 
-    def create_dossier_with_pj_of_size(size, procedure)
-      dossier = create(:dossier, :accepte, procedure: procedure)
-      create(:champ_piece_justificative, size: size, dossier: dossier)
-    end
+  def create_dossier_with_pj_of_size(size, procedure)
+    dossier = create(:dossier, :accepte, procedure: procedure)
+    create(:champ_piece_justificative, size: size, dossier: dossier)
+  end
+
+  def create_dossier(construction_date:, instruction_date:, processed_date:)
+    dossier = create(:dossier, :accepte, procedure: procedure, en_construction_at: construction_date, en_instruction_at: instruction_date, processed_at: processed_date)
   end
 end