diff --git a/app/models/procedure.rb b/app/models/procedure.rb
index 80e5629e7..8f9c31861 100644
--- a/app/models/procedure.rb
+++ b/app/models/procedure.rb
@@ -405,7 +405,7 @@ class Procedure < ApplicationRecord
     times = dossiers
       .state_termine
       .pluck(start_attribute, end_attribute)
-      .map { |times| times[1] - times[0] }
+      .map { |(start_date, end_date)| end_date - start_date }
 
     if times.present?
       times.sum.fdiv(times.size).ceil
diff --git a/lib/tasks/2018_09_27_fill_missing_en_instruction_at.rake b/lib/tasks/2018_09_27_fill_missing_en_instruction_at.rake
new file mode 100644
index 000000000..e100e11ef
--- /dev/null
+++ b/lib/tasks/2018_09_27_fill_missing_en_instruction_at.rake
@@ -0,0 +1,9 @@
+namespace :'2018_09_27_fill_missing_en_instruction_at' do
+  task run: :environment do
+    dossiers_with_missing_instruction_at = Dossier
+      .where.not(processed_at: nil)
+      .where(en_instruction_at: nil)
+
+    dossiers_with_missing_instruction_at.each { |d| d.update(en_instruction_at: d.processed_at) }
+  end
+end
diff --git a/spec/models/procedure_spec.rb b/spec/models/procedure_spec.rb
index d41f2413a..9642789c0 100644
--- a/spec/models/procedure_spec.rb
+++ b/spec/models/procedure_spec.rb
@@ -773,4 +773,23 @@ describe Procedure do
       expect(p.juridique_required).to be_truthy
     end
   end
+
+  describe '#mean_instruction_time' do
+    let(:procedure) { create(:procedure) }
+
+    context 'when there is only one dossier' do
+      let(:dossier) { create(:dossier, procedure: procedure) }
+
+      context 'which is termine' do
+        before do
+          dossier.accepte!
+          processed_date = DateTime.parse('12/12/2012')
+          instruction_date = processed_date - 1.day
+          dossier.update(en_instruction_at: instruction_date, processed_at: processed_date)
+        end
+
+        it { expect(procedure.mean_instruction_time).to eq(1.day.to_i) }
+      end
+    end
+  end
 end