Note: this list is generated by looking for the string for this theme hook, so it may include some references that are not actually using this theme hook.

backdrop_common_theme in core/includes/common.inc
Provides theme registration for themes across .inc files.
CommonFormatDateTestCase::testFormatDate in core/modules/simpletest/tests/common.test
Tests for the format_date() function.
form_test_h5datetime in core/modules/simpletest/tests/form_test.module
Form constructor for #type html_date, html_time and html_datetime elements.
node_form in core/modules/node/node.pages.inc
Form constructor for the node add/edit form.
system_element_info in core/modules/system/system.module
Implements hook_element_info().
system_update_1020 in core/modules/system/system.install
Convert date settings to config.
template_preprocess_datetime in core/includes/theme.inc
Preprocess variables for theme_datetime().