Parse date in NotificationPayloadParser

Use `DateTime.parse` to support dates of various formats

ee/lib/gitlab/alerting/notification_payload_parser.rb

@@ -42,12 +42,14 @@ module Gitlab
         payload[:hosts] && Array(payload[:hosts])
       end
 
-      def starts_at
-        payload[:start_time].presence || current_time
+      def current_time
+        Time.current.change(usec: 0).rfc3339
       end
 
-      def current_time
-        Time.now.change(usec: 0).rfc3339
+      def starts_at
+        DateTime.parse(payload[:start_time].to_s).rfc3339
+      rescue ArgumentError
+        current_time
       end
     end
   end

ee/spec/lib/gitlab/alerting/notification_payload_parser_spec.rb

@@ -4,7 +4,7 @@ require 'fast_spec_helper'
 
 describe Gitlab::Alerting::NotificationPayloadParser do
   describe '.call' do
-    let(:starts_at) { Time.now.change(usec: 0) }
+    let(:starts_at) { Time.current.change(usec: 0) }
     let(:payload) do
       {
         'title' => 'alert title',
@@ -53,6 +53,16 @@ describe Gitlab::Alerting::NotificationPayloadParser do
       end
     end
 
+    context 'when the time is in unsupported format' do
+      before do
+        payload[:start_time] = 'invalid/date/format'
+      end
+
+      it 'sets startsAt to a currurrent time in RFC3339 format' do
+        expect(subject['startsAt']).to eq(starts_at.rfc3339)
+      end
+    end
+
     context 'when payload is blank' do
       let(:payload) { {} }