11 files declare their use of DateTimePlus in Drupal 9

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

Date.php in core/modules/datetime/src/Plugin/views/filter/Date.php

DateTimeFormatConstraintValidator.php in core/modules/datetime/src/Plugin/Validation/Constraint/DateTimeFormatConstraintValidator.php

DateTimeHandlerTestBase.php in core/modules/datetime/tests/src/Kernel/Views/DateTimeHandlerTestBase.php

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

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

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

EntitySerializationTest.php in core/modules/serialization/tests/src/Kernel/EntitySerializationTest.php

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

FormatDate.php in core/modules/migrate/src/Plugin/migrate/process/FormatDate.php

PageCacheTest.php in core/modules/page_cache/tests/src/Functional/PageCacheTest.php