Fix timer timezone handling

app/controllers/tournaments_controller.rb

@@ -96,7 +96,7 @@ class TournamentsController < ApplicationController
 
   # GET /tournaments/:id/timer_end
   def timer_end
-    render json: { timer_end: @tournament.timer_end }
+    render json: { timer_end: serialize_timer_end(@tournament.timer_end) }
   end
 
   # PATCH /tournaments/:id/set_timer_end
@@ -115,6 +115,10 @@ class TournamentsController < ApplicationController
     { timer_end: params[:timer_end] }
   end
 
+  def serialize_timer_end(timer_end)
+    timer_end&.iso8601
+  end
+
   def organize_teams_in_groups(teams)
     # each team gets put into an array of teams depending on the group specified in team[:group]
     teams.group_by { |team| team['group'] }.values.map do |group|
@@ -177,9 +181,8 @@ class TournamentsController < ApplicationController
       error: 'playoff_teams_amount, instant_finalists_amount and intermediate_round_participants_amount don\'t match'
     }, status: :unprocessable_entity
   end
-end
 
-def validate_set_timer_end_params
+  def validate_set_timer_end_params
     timer_end = params[:timer_end]
     timer_end_seconds = params[:timer_end_seconds]
 
@@ -207,10 +210,13 @@ def validate_set_timer_end_params
         elsif !parsed_time.future?
           return render json: { error: 'Timer end must be in the future' }, status: :unprocessable_entity
         end
+
+        params[:timer_end] = parsed_time
       rescue ArgumentError
         return render json: { error: 'Invalid datetime format' }, status: :unprocessable_entity
       end
     else
       return render json: { error: 'Timer end is required' }, status: :unprocessable_entity
     end
+  end
 end

app/serializers/tournament_serializer.rb

@@ -2,9 +2,13 @@
 
 class TournamentSerializer < SimpleTournamentSerializer
   attributes :description, :playoff_teams_amount,
-             :instant_finalists_amount, :intermediate_round_participants_amount, :timer_end
+             :instant_finalists_amount, :intermediate_round_participants_amount
   has_many :stages
 
+  attribute :timer_end do
+    object.timer_end&.iso8601
+  end
+
   attribute :owner_username do
     object.owner.username
   end

spec/controllers/tournaments_controller_spec.rb

@@ -142,6 +142,34 @@ RSpec.describe TournamentsController, type: :controller do
     end
   end
 
+  describe 'GET #timer_end' do
+    it 'returns timer_end in ISO 8601 format' do
+      timer_end = Time.utc(2026, 4, 13, 18, 0, 0)
+      @tournament.update!(timer_end: timer_end)
+
+      get :timer_end, params: { id: @tournament.to_param }
+
+      expect(response).to be_successful
+      expect(deserialize_response(response)[:timer_end]).to eq(timer_end.iso8601)
+    end
+  end
+
+  describe 'PATCH #set_timer_end' do
+    before(:each) do
+      apply_authentication_headers_for @user
+    end
+
+    it 'stores timezone-aware timer_end values without shifting them' do
+      timer_end = '2030-04-13T18:00:00+02:00'
+
+      patch :set_timer_end, params: { id: @tournament.to_param, timer_end: timer_end }
+
+      expect(response).to be_successful
+      expect(@tournament.reload.timer_end.iso8601).to eq(Time.iso8601(timer_end).utc.iso8601)
+      expect(deserialize_response(response)[:timer_end]).to eq(@tournament.timer_end.iso8601)
+    end
+  end
+
   describe 'POST #create' do
     let(:create_playoff_tournament_data) do
       {