fix(stat): enforce time travel when dealing with relative dates

spec/models/stat_spec.rb

@@ -1,3 +1,5 @@
+include ActiveSupport::Testing::TimeHelpers
+
 describe Stat do
   describe '.deleted_dossiers_states' do
     subject { Stat.send(:deleted_dossiers_states) }
@@ -78,6 +80,7 @@ describe Stat do
 
   describe '.last_four_months_hash' do
     it 'works count and cumulate counters by month for both dossier and deleted dossiers' do
+      travel_to Time.zone.local(2021, 11, 25) do
         4.downto(1).map do |i|
           create(:dossier, state: :en_construction, en_construction_at: i.months.ago)
           create(:deleted_dossier, dossier_id: i + 100, state: :en_construction, deleted_at: i.month.ago)
@@ -93,6 +96,7 @@ describe Stat do
         ])
       end
     end
+  end
 
   describe '.sum_hashes' do
     it 'sum up hashes keys' do