6 files declare their use of DateTimePlus in Zircon Profile 8.0

AggregatorTestRssController.php in core/modules/aggregator/tests/modules/aggregator_test/src/Controller/AggregatorTestRssController.php

Contains \Drupal\aggregator_test\Controller\AggregatorTestRssController.

DateTimePlusTest.php in core/tests/Drupal/Tests/Component/Datetime/DateTimePlusTest.php

Contains \Drupal\Tests\Component\Datetime\DateTimePlusTest.

DefaultFetcher.php in core/modules/aggregator/src/Plugin/aggregator/fetcher/DefaultFetcher.php

Contains \Drupal\aggregator\Plugin\aggregator\fetcher\DefaultFetcher.

DrupalDateTime.php in core/lib/Drupal/Core/Datetime/DrupalDateTime.php

Contains \Drupal\Core\Datetime\DrupalDateTime.

FinishResponseSubscriber.php in core/lib/Drupal/Core/EventSubscriber/FinishResponseSubscriber.php

Contains \Drupal\Core\EventSubscriber\FinishResponseSubscriber.

PageCacheTest.php in core/modules/page_cache/src/Tests/PageCacheTest.php

Contains \Drupal\page_cache\Tests\PageCacheTest.