diff --git a/app/models/procedure.rb b/app/models/procedure.rb
index 44984c564..8242e67c4 100644
--- a/app/models/procedure.rb
+++ b/app/models/procedure.rb
@@ -426,6 +426,7 @@ class Procedure < ApplicationRecord
   def percentile_time(start_attribute, end_attribute, p)
     times = dossiers
       .state_termine
+      .where(end_attribute => 1.month.ago..DateTime.current)
       .pluck(start_attribute, end_attribute)
       .map { |(start_date, end_date)| end_date - start_date }
 
diff --git a/spec/models/procedure_spec.rb b/spec/models/procedure_spec.rb
index c87a32593..0f26cdb33 100644
--- a/spec/models/procedure_spec.rb
+++ b/spec/models/procedure_spec.rb
@@ -727,6 +727,15 @@ describe Procedure do
       end
     end
 
+    context 'when there are very old dossiers' do
+      let(:processed_delays) { [2.days, 2.days] }
+      let!(:old_dossier) { create_dossier(instruction_date: 3.months.ago, processed_date: 2.months.ago) }
+
+      it 'ignores dossiers older than 1 month' do
+        expect(procedure.usual_instruction_time).to be_within(10.seconds).of(2.days)
+      end
+    end
+
     context 'when there is only one processed dossier' do
       let(:processed_delays) { [1.day] }
       it { expect(procedure.usual_instruction_time).to be_within(10.seconds).of(1.day) }