20 files declare their use of DateFormat in Drupal 8

comment.tokens.inc in core/modules/comment/comment.tokens.inc

Builds placeholder replacement tokens for comment-related data.

CommentOrphanTest.php in core/modules/comment/tests/src/Kernel/CommentOrphanTest.php

DateFormatAccessControlHandlerTest.php in core/modules/system/tests/src/Kernel/DateFormatAccessControlHandlerTest.php

DateFormatResourceTestBase.php in core/tests/Drupal/FunctionalTests/Rest/DateFormatResourceTestBase.php

DateFormatTest.php in core/modules/jsonapi/tests/src/Functional/DateFormatTest.php

DateFormatTest.php in core/modules/system/tests/src/FunctionalJavascript/System/DateFormatTest.php

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

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

DateTimeFieldTest.php in core/modules/datetime/tests/src/Functional/DateTimeFieldTest.php

DateTimeTest.php in core/modules/system/tests/src/Functional/System/DateTimeTest.php

file.module in core/modules/file/file.module

Defines a "managed_file" Form API field and a "file" field for Field module.

MigrateDateFormatTest.php in core/modules/system/tests/src/Kernel/Migrate/d6/MigrateDateFormatTest.php

node.tokens.inc in core/modules/node/node.tokens.inc

Builds placeholder replacement tokens for node-related data.

SummaryLengthTest.php in core/modules/node/tests/src/Kernel/SummaryLengthTest.php

system.tokens.inc in core/modules/system/system.tokens.inc

Builds placeholder replacement tokens system-wide data.

TimestampDatetimeWidget.php in core/lib/Drupal/Core/Datetime/Plugin/Field/FieldWidget/TimestampDatetimeWidget.php

TimestampTest.php in core/tests/Drupal/FunctionalTests/Datetime/TimestampTest.php

TimezoneTest.php in core/tests/Drupal/KernelTests/Core/Datetime/Element/TimezoneTest.php

user.tokens.inc in core/modules/user/user.tokens.inc

Builds placeholder replacement tokens for user-related data.

UserTimeZoneTest.php in core/modules/user/tests/src/Functional/UserTimeZoneTest.php