Correct time zone of API dates

app/serializers/event_schedule_serializer.rb

@@ -6,8 +6,7 @@ class EventScheduleSerializer < ActiveModel::Serializer
   attributes :date, :room
 
   def date
-    t = object.start_time
-    t.blank? ? '' : %( #{I18n.l t, format: :short}#{t.formatted_offset(false)} )
+    object.start_time&.change(zone: object.event.program.conference.timezone)
   end
 
   def room

app/serializers/event_serializer.rb

@@ -6,8 +6,7 @@ class EventSerializer < ActiveModel::Serializer
   attributes :guid, :title, :length, :scheduled_date, :language, :abstract, :speaker_ids, :type, :room, :track
 
   def scheduled_date
-    t = object.time
-    t.blank? ? '' : %( #{I18n.l t, format: :short}#{t.formatted_offset(false)} )
+    object.time&.change(zone: object.program.conference.timezone)
   end
 
   def speaker_ids

spec/serializers/event_schedule_serializer_spec.rb

@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe EventScheduleSerializer, type: :serializer do
+  let(:start) { DateTime.new(2000, 1, 2, 3, 4, 5) }
+  let(:timezone) { 'Etc/GMT+11' }
+  let(:conference) do
+    create(:conference, start_date: start.to_date,
+                        start_hour: start.hour,
+                        timezone:   timezone)
+  end
+  let(:program) { create(:program, conference: conference) }
+  let(:event) { create(:event, program: program) }
+  let(:event_schedule) { create(:event_schedule, event: event, start_time: start) }
+  let(:serializer) { described_class.new(event_schedule) }
+
+  it 'sets date and room' do
+    expected_json = {
+      date: '2000-01-02T03:04:05.000-11:00',
+      room: event_schedule.room.guid
+    }.to_json
+
+    expect(serializer.to_json).to eq expected_json
+  end
+end

spec/serializers/event_serializer_spec.rb

@@ -2,7 +2,13 @@
 
 require 'spec_helper'
 describe EventSerializer, type: :serializer do
-  let(:event) { create(:event, title: 'Some Talk', abstract: 'Lorem ipsum dolor sit amet') }
+  let(:conference) { create(:conference, timezone: 'UTC') }
+  let(:program) { create(:program, conference: conference) }
+  let(:event) do
+    create(:event, program:  program,
+                   title:    'Some Talk',
+                   abstract: 'Lorem ipsum dolor sit amet')
+  end
   let(:serializer) { EventSerializer.new(event) }
 
   context 'event does not have date, room and tracks assigned' do
@@ -11,7 +17,7 @@
         guid:           event.guid,
         title:          'Some Talk',
         length:         30,
-        scheduled_date: '',
+        scheduled_date: nil,
         language:       nil,
         abstract:       'Lorem ipsum dolor sit amet',
         speaker_ids:    event.speaker_ids,
@@ -41,7 +47,7 @@
         guid:           event.guid,
         title:          'Some Talk',
         length:         30,
-        scheduled_date: ' 2014-03-04T09:00:00+0000 ',
+        scheduled_date: '2014-03-04T09:00:00.000Z',
         language:       'English',
         abstract:       'Lorem ipsum dolor sit amet',
         speaker_ids:    [speaker.id],