18 files declare their use of DateTimeItemInterface in Drupal 8

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

DateField.php in core/modules/datetime/src/Plugin/migrate/field/DateField.php

DateRangeFieldItemList.php in core/modules/datetime_range/src/Plugin/Field/FieldType/DateRangeFieldItemList.php

DateRangeFieldTest.php in core/modules/datetime_range/tests/src/Functional/DateRangeFieldTest.php

DateRangeItem.php in core/modules/datetime_range/src/Plugin/Field/FieldType/DateRangeItem.php

DateRangeWidgetBase.php in core/modules/datetime_range/src/Plugin/Field/FieldWidget/DateRangeWidgetBase.php

DateTimeComputed.php in core/modules/datetime/src/DateTimeComputed.php

DateTimeCustomFormatter.php in core/modules/datetime/src/Plugin/Field/FieldFormatter/DateTimeCustomFormatter.php

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

DateTimeIso8601Normalizer.php in core/modules/serialization/src/Normalizer/DateTimeIso8601Normalizer.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

FilterDateTest.php in core/modules/datetime/tests/src/Functional/Views/FilterDateTest.php

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