Fix convert timezone time

- We needed to ensure we convert at todays date

ee/app/services/incident_management/oncall_schedules/update_service.rb

@@ -41,8 +41,8 @@ module IncidentManagement
             rotation,
             user,
             {
-              active_period_start: new_rotation_active_period(rotation.active_period_start).strftime('%H:%M'),
-              active_period_end: new_rotation_active_period(rotation.active_period_end).strftime('%H:%M')
+              active_period_start: new_rotation_active_period(rotation.active_period_start),
+              active_period_end: new_rotation_active_period(rotation.active_period_end)
             }
           )
 
@@ -52,8 +52,12 @@ module IncidentManagement
         end
       end
 
-      def new_rotation_active_period(time_string)
-        time_string.in_time_zone(original_schedule_timezone).in_time_zone(oncall_schedule.timezone)
+      def new_rotation_active_period(existing_time)
+        existing_timezone_time = Time.find_zone!(original_schedule_timezone)
+                                  .now
+                                  .change(hour: existing_time.hour, min: existing_time.min)
+
+        existing_timezone_time.in_time_zone(oncall_schedule.timezone).strftime('%H:%M')
       end
 
       def error_no_permissions

ee/spec/services/incident_management/oncall_schedules/update_service_spec.rb

@@ -6,7 +6,7 @@ RSpec.describe IncidentManagement::OncallSchedules::UpdateService do
   let_it_be(:user_with_permissions) { create(:user) }
   let_it_be(:user_without_permissions) { create(:user) }
   let_it_be_with_refind(:project) { create(:project) }
-  let_it_be_with_reload(:oncall_schedule) { create(:incident_management_oncall_schedule, project: project) }
+  let_it_be_with_reload(:oncall_schedule) { create(:incident_management_oncall_schedule, :utc, project: project) }
 
   let(:current_user) { user_with_permissions }
   let(:params) { { name: 'Updated name', description: 'Updated description', timezone: 'America/New_York' } }
@@ -81,9 +81,10 @@ RSpec.describe IncidentManagement::OncallSchedules::UpdateService do
 
         let_it_be_with_reload(:oncall_rotation) { create(:incident_management_oncall_rotation, :with_active_period, schedule: oncall_schedule) }
 
+        # This expects the active periods are updated according to the date above (22nd March, 2021 in the new timezone).
         it 'updates the rotation active periods with new timezone' do
-          expect { execute }.to change { time_from_time_column(oncall_rotation.reload.active_period_start) }.from('08:00').to('03:00')
-           .and change { time_from_time_column(oncall_rotation.active_period_end) }.from('17:00').to('12:00')
+          expect { execute }.to change { time_from_time_column(oncall_rotation.reload.active_period_start) }.from('08:00').to('04:00')
+           .and change { time_from_time_column(oncall_rotation.active_period_end) }.from('17:00').to('13:00')
         end
 
         context 'error updating' do