24 files declare their use of DateTimeItem

Date.php in core/modules/datetime/src/Plugin/views/argument/Date.php
Date.php in core/modules/datetime/src/Plugin/views/filter/Date.php
Date.php in core/modules/datetime/src/Plugin/views/sort/Date.php
DateFilterTest.php in core/modules/datetime/tests/src/Functional/DateFilterTest.php
DateRangeItem.php in core/modules/datetime_range/src/Plugin/Field/FieldType/DateRangeItem.php
DateTestBase.php in core/modules/datetime/tests/src/Functional/DateTestBase.php
DateTestBase.php in core/modules/datetime/src/Tests/DateTestBase.php
DateTimeComputed.php in core/modules/datetime/src/DateTimeComputed.php
DateTimeDefaultWidget.php in core/modules/datetime/src/Plugin/Field/FieldWidget/DateTimeDefaultWidget.php
DateTimeFormatConstraintValidator.php in core/modules/datetime/src/Plugin/Validation/Constraint/DateTimeFormatConstraintValidator.php
DateTimeFormatterBase.php in core/modules/datetime/src/Plugin/Field/FieldFormatter/DateTimeFormatterBase.php
DateTimeHandlerTestBase.php in core/modules/datetime/src/Tests/Views/DateTimeHandlerTestBase.php
DateTimeHandlerTestBase.php in core/modules/datetime/tests/src/Kernel/Views/DateTimeHandlerTestBase.php
DateTimeIso8601Normalizer.php in core/modules/serialization/src/Normalizer/DateTimeIso8601Normalizer.php
DateTimeIso8601NormalizerTest.php in core/modules/serialization/tests/src/Unit/Normalizer/DateTimeIso8601NormalizerTest.php
DateTimeItemTest.php in core/modules/datetime/tests/src/Kernel/DateTimeItemTest.php
DateTimePlainFormatter.php in core/modules/datetime/src/Plugin/Field/FieldFormatter/DateTimePlainFormatter.php
DateTimeWidgetBase.php in core/modules/datetime/src/Plugin/Field/FieldWidget/DateTimeWidgetBase.php
EntityTestDateonlyTest.php in core/modules/datetime/tests/src/Functional/EntityResource/EntityTest/EntityTestDateonlyTest.php
EntityTestDatetimeTest.php in core/modules/datetime/tests/src/Functional/EntityResource/EntityTest/EntityTestDatetimeTest.php
FilterDateTest.php in core/modules/datetime/tests/src/Kernel/Views/FilterDateTest.php
FilterDateTest.php in core/modules/datetime/tests/src/Functional/Views/FilterDateTest.php
JsonApiRegressionTest.php in core/modules/jsonapi/tests/src/Functional/JsonApiRegressionTest.php
SeparatorTranslationTest.php in core/modules/datetime_range/tests/src/Kernel/SeparatorTranslationTest.php

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