Ensure datetime/date are serialized to correct format (#33086)

spec/serializers/rest/admin/measure_serializer_spec.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe REST::Admin::MeasureSerializer do
+  subject { serialized_record_json(record, described_class) }
+
+  let(:start_at) { 2.days.ago }
+  let(:end_at) { Time.now.utc }
+  let(:params) { ActionController::Parameters.new({ instance_accounts: [123] }) }
+  let(:record) { Admin::Metrics::Measure::ActiveUsersMeasure.new(start_at, end_at, params) }
+
+  context 'when start_at is populated' do
+    it 'parses as RFC 3339 datetime' do
+      subject['data'].each { |datum| expect { DateTime.rfc3339(datum['date']) }.to_not raise_error }
+    end
+  end
+end