8 files declare their use of DateFormat in Zircon Profile 8.0

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

Builds placeholder replacement tokens for comment-related data.

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

Contains \Drupal\Core\Datetime\Element\Datetime.

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/src/Tests/Migrate/d6/MigrateDateFormatTest.php

Contains \Drupal\system\Tests\Migrate\d6\MigrateDateFormatTest.

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

Builds placeholder replacement tokens for node-related data.

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

Contains \Drupal\Core\Datetime\Plugin\Field\FieldWidget\TimestampDatetimeWidget.

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

Builds placeholder replacement tokens for user-related data.