diff --git a/src/wp-admin/options-general.php b/src/wp-admin/options-general.php
index 969065b7008f4..c5e00b09ab3cf 100644
--- a/src/wp-admin/options-general.php
+++ b/src/wp-admin/options-general.php
@@ -486,18 +486,53 @@ class="<?php echo esc_attr( $classes_for_button ); ?>"
 <td>
 	<fieldset><legend class="screen-reader-text"><span><?php echo $date_format_title; ?></span></legend>
 <?php
-	/**
-	 * Filters the default date formats.
-	 *
-	 * @since 2.7.0
-	 * @since 4.0.0 Replaced the `Y/m/d` format with `Y-m-d` (ISO date standard YYYY-MM-DD).
-	 * @since 6.8.0 Added the `d.m.Y` format.
-	 *
-	 * @param string[] $default_date_formats Array of default date formats.
-	 */
-	$date_formats = array_unique( apply_filters( 'date_formats', array( __( 'F j, Y' ), 'Y-m-d', 'm/d/Y', 'd/m/Y', 'd.m.Y' ) ) );
+/**
+ * Determine which format string is the default for the site's configured locale.
+ *
+ * get_locale()       always returns the site locale (WPLANG option/constant).
+ * determine_locale() returns the user locale in admin contexts.
+ *
+ * We briefly switch to the site locale only when the two differ and
+ * the language pack is installed. If the switch fails (pack missing),
+ * $site_locale_date_default stays null and no label is shown — that is
+ * intentionally safe: we never guess.
+ *
+ * @since 7.1.0
+ * @ticket 64102
+ */
+$site_locale    = get_locale();
+$current_locale = determine_locale();
 
-	$custom = true;
+$site_locale_date_default = null;
+$site_locale_time_default = null;
+
+if ( $site_locale === $current_locale ) {
+	// Already running in the site locale — __() gives the correct translation.
+	$site_locale_date_default = __( 'F j, Y' );
+	$site_locale_time_default = __( 'g:i a' );
+} else {
+	// User locale differs from site locale. switch_to_locale() returns false
+	// when the language pack isn't installed on disk, keeping both vars null.
+	$locale_switched = switch_to_locale( $site_locale );
+	if ( $locale_switched ) {
+		$site_locale_date_default = __( 'F j, Y' );
+		$site_locale_time_default = __( 'g:i a' );
+		restore_previous_locale();
+	}
+}
+
+/**
+ * Filters the default date formats.
+ *
+ * @since 2.7.0
+ * @since 4.0.0 Replaced the `Y/m/d` format with `Y-m-d` (ISO date standard YYYY-MM-DD).
+ * @since 6.8.0 Added the `d.m.Y` format.
+ *
+ * @param string[] $default_date_formats Array of default date formats.
+ */
+$date_formats = array_unique( apply_filters( 'date_formats', array( __( 'F j, Y' ), 'Y-m-d', 'm/d/Y', 'd/m/Y', 'd.m.Y' ) ) );
+
+$custom = true;
 
 foreach ( $date_formats as $format ) {
 	echo "\t<label><input type='radio' name='date_format' value='" . esc_attr( $format ) . "'";
@@ -505,7 +540,12 @@ class="<?php echo esc_attr( $classes_for_button ); ?>"
 		echo " checked='checked'";
 		$custom = false;
 	}
-	echo ' /> <span class="date-time-text format-i18n">' . date_i18n( $format ) . '</span><code>' . esc_html( $format ) . "</code></label><br />\n";
+	echo ' /> <span class="date-time-text format-i18n">' . date_i18n( $format ) . '</span><code>' . esc_html( $format ) . '</code>';
+	if ( null !== $site_locale_date_default && $format === $site_locale_date_default ) {
+		/* translators: Shown next to the date/time format that is the default for the site's language. */
+		echo ' <span class="description">' . __( '(Language default)' ) . '</span>';
+	}
+	echo "</label><br />\n";
 }
 
 	echo '<label><input type="radio" name="date_format" id="date_format_custom_radio" value="\c\u\s\t\o\m"';
@@ -550,7 +590,12 @@ class="<?php echo esc_attr( $classes_for_button ); ?>"
 		echo " checked='checked'";
 		$custom = false;
 	}
-	echo ' /> <span class="date-time-text format-i18n">' . date_i18n( $format ) . '</span><code>' . esc_html( $format ) . "</code></label><br />\n";
+	echo ' /> <span class="date-time-text format-i18n">' . date_i18n( $format ) . '</span><code>' . esc_html( $format ) . '</code>';
+	if ( null !== $site_locale_time_default && $format === $site_locale_time_default ) {
+		/* translators: Shown next to the date/time format that is the default for the site's language. */
+		echo ' <span class="description">' . __( '(Language default)' ) . '</span>';
+	}
+	echo "</label><br />\n";
 }
 
 	echo '<label><input type="radio" name="time_format" id="time_format_custom_radio" value="\c\u\s\t\o\m"';
diff --git a/tests/e2e/specs/general-settings-date-time-format.test.js b/tests/e2e/specs/general-settings-date-time-format.test.js
new file mode 100644
index 0000000000000..bf69a1dabffd8
--- /dev/null
+++ b/tests/e2e/specs/general-settings-date-time-format.test.js
@@ -0,0 +1,42 @@
+/**
+ * E2E tests for the "(Language default)" label on Settings > General.
+ *
+ * @ticket 64102
+ */
+import { test, expect } from '@wordpress/e2e-test-utils-playwright';
+
+test.describe( 'Settings -> General: Date/Time format language default label', () => {
+	test.beforeEach( async ( { admin } ) => {
+		await admin.visitAdminPage( '/options-general.php' );
+	} );
+
+	test( 'Shows "(Language default)" label next to exactly one date format option', async ( { page } ) => {
+		const labels = page.locator( 'input[name="date_format"] ~ .description' );
+		await expect( labels.filter( { hasText: 'Language default' } ) ).toHaveCount( 1 );
+	} );
+
+	test( 'Shows "(Language default)" label next to exactly one time format option', async ( { page } ) => {
+		const labels = page.locator( 'input[name="time_format"] ~ .description' );
+		await expect( labels.filter( { hasText: 'Language default' } ) ).toHaveCount( 1 );
+	} );
+
+	test( 'Date format preview text is not affected by the language default label', async ( { page } ) => {
+		// Click the first (non-custom) date format radio.
+		await page.locator( 'input[name="date_format"]' ).first().click();
+		// The .example span should not include the label text.
+		await expect( page.locator( 'fieldset:has(input[name="date_format"]) .example' ) ).not.toContainText( 'Language default' );
+	} );
+
+	test( 'Time format preview text is not affected by the language default label', async ( { page } ) => {
+		await page.locator( 'input[name="time_format"]' ).first().click();
+		await expect( page.locator( 'fieldset:has(input[name="time_format"]) .example' ) ).not.toContainText( 'Language default' );
+	} );
+
+	test( 'Language default label is not inside the .format-i18n span', async ( { page } ) => {
+		const formatI18nSpans = page.locator( '.format-i18n' );
+		const count = await formatI18nSpans.count();
+		for ( let i = 0; i < count; i++ ) {
+			await expect( formatI18nSpans.nth( i ) ).not.toContainText( 'Language default' );
+		}
+	} );
+} );
diff --git a/tests/phpunit/tests/admin/optionsGeneral.php b/tests/phpunit/tests/admin/optionsGeneral.php
new file mode 100644
index 0000000000000..dae680bdfdd49
--- /dev/null
+++ b/tests/phpunit/tests/admin/optionsGeneral.php
@@ -0,0 +1,382 @@
+<?php
+/**
+ * Tests for the "(Language default)" label in the Date/Time format section
+ * of Settings > General (options-general.php).
+ *
+ * The locale-discovery block at the top of that section captures the site
+ * locale's default date/time format strings. These tests verify the logic
+ * of that block in isolation, without rendering the full admin page.
+ *
+ * @ticket 64102
+ * @group  admin
+ * @group  options-general
+ * @group  l10n
+ */
+class Tests_Admin_Options_General_DateTimeLabel extends WP_UnitTestCase {
+
+	/**
+	 * Administrator user created once for all tests in this class.
+	 *
+	 * @var int
+	 */
+	protected static $admin_id;
+
+	public static function wpSetUpBeforeClass( WP_UnitTest_Factory $factory ) {
+		self::$admin_id = $factory->user->create(
+			array(
+				'role'   => 'administrator',
+				'locale' => '', // No explicit user locale — will be overridden per test.
+			)
+		);
+	}
+
+	public function set_up() {
+		parent::set_up();
+
+		// Clear translation caches so locale switches take effect cleanly.
+		unset( $GLOBALS['l10n'], $GLOBALS['l10n_unloaded'] );
+	}
+
+	public function tear_down() {
+		// Unwind any locale switch that a test may have left active.
+		restore_current_locale();
+
+		// Clear translation caches.
+		unset( $GLOBALS['l10n'], $GLOBALS['l10n_unloaded'] );
+
+		// Restore the $locale global to whatever it was before this test manipulated it.
+		parent::tear_down();
+	}
+
+	// -------------------------------------------------------------------------
+	// Helper
+	// -------------------------------------------------------------------------
+
+	/**
+	 * Replicates the locale-discovery block from options-general.php.
+	 *
+	 * Returns the site locale's default date/time format strings, or null for
+	 * each when the site locale's language pack is not installed on disk (so
+	 * the caller never shows a label it cannot reliably identify).
+	 *
+	 * Note: the null-guard (switch path) only fires when the site locale and the
+	 * user/current locale are DIFFERENT. When they are the same, the fast path
+	 * takes over — and if the locale is invalid/unloaded, __() still returns the
+	 * English string, which is acceptable because the page is already rendering in
+	 * that same (site) locale context.
+	 *
+	 * @return array{ date: string|null, time: string|null }
+	 */
+	private function get_site_locale_defaults() {
+		$site_locale    = get_locale();
+		$current_locale = determine_locale();
+
+		$site_locale_date_default = null;
+		$site_locale_time_default = null;
+
+		if ( $site_locale === $current_locale ) {
+			// Fast path: already in the site locale — __() gives the correct value.
+			$site_locale_date_default = __( 'F j, Y' );
+			$site_locale_time_default = __( 'g:i a' );
+		} else {
+			// switch_to_locale() returns false when the pack is not installed.
+			$locale_switched = switch_to_locale( $site_locale );
+			if ( $locale_switched ) {
+				$site_locale_date_default = __( 'F j, Y' );
+				$site_locale_time_default = __( 'g:i a' );
+				restore_previous_locale();
+			}
+		}
+
+		return array(
+			'date' => $site_locale_date_default,
+			'time' => $site_locale_time_default,
+		);
+	}
+
+	// -------------------------------------------------------------------------
+	// Tests: locale-discovery logic
+	// -------------------------------------------------------------------------
+
+	/**
+	 * Fast path: when site locale and user locale are the same, __() already
+	 * runs in the correct locale — no switch needed and defaults are captured.
+	 *
+	 * @ticket 64102
+	 * @covers ::determine_locale
+	 * @covers ::get_locale
+	 */
+	public function test_defaults_captured_when_site_and_user_locale_match() {
+		// Default test environment: site = en_US, user = en_US (no override).
+		// get_locale() and determine_locale() both return 'en_US'.
+		$defaults = $this->get_site_locale_defaults();
+
+		$this->assertSame(
+			'F j, Y',
+			$defaults['date'],
+			'Date default must be captured when site and user locale are the same.'
+		);
+		$this->assertSame(
+			'g:i a',
+			$defaults['time'],
+			'Time default must be captured when site and user locale are the same.'
+		);
+	}
+
+	/**
+	 * Switch path: en_US is hardcoded into WP_Locale_Switcher::$available_languages,
+	 * so switch_to_locale( 'en_US' ) always succeeds. The site locale defaults are
+	 * captured even when the user locale differs.
+	 *
+	 * @ticket 64102
+	 * @covers ::switch_to_locale
+	 * @covers ::restore_previous_locale
+	 */
+	public function test_defaults_captured_when_site_is_en_us_and_user_locale_differs() {
+		global $locale;
+		$old_locale = $locale;
+
+		// Set site locale to en_US directly via the global (bypasses DB reads,
+		// avoids option-cache invalidation complexity).
+		$locale = 'en_US';
+
+		// Force determine_locale() to return a locale different from en_US so the
+		// switch path is taken. Priority 0 runs before the locale switcher (priority 10)
+		// but is correctly overridden BY the switcher while a switch is in progress.
+		$override = static function () {
+			return 'de_DE';
+		};
+		add_filter( 'determine_locale', $override, 0 );
+
+		$defaults = $this->get_site_locale_defaults();
+
+		remove_filter( 'determine_locale', $override, 0 );
+		$locale = $old_locale;
+
+		// en_US is always in WP_Locale_Switcher::$available_languages (hardcoded in
+		// the constructor). The switch must therefore succeed and defaults be captured.
+		$this->assertSame(
+			'F j, Y',
+			$defaults['date'],
+			'Site locale default must be captured when site=en_US and user locale differs.'
+		);
+		$this->assertSame(
+			'g:i a',
+			$defaults['time'],
+			'Site locale time default must be captured when site=en_US and user locale differs.'
+		);
+	}
+
+	/**
+	 * Null guard: when the site locale string is not a known/installed locale,
+	 * switch_to_locale() returns false, and both defaults remain null.
+	 * No misleading label must ever be shown.
+	 *
+	 * @ticket 64102
+	 * @covers ::switch_to_locale
+	 */
+	public function test_defaults_are_null_when_site_locale_pack_not_installed() {
+		global $locale;
+		$old_locale = $locale;
+
+		// Site locale = 'zz_ZZ': a locale that does not exist on disk.
+		$locale = 'zz_ZZ';
+
+		// Force determine_locale() to return 'en_US' so that site ≠ current locale,
+		// which forces the switch path. The switch then fails because 'zz_ZZ' is not
+		// in WP_Locale_Switcher::$available_languages, keeping defaults null.
+		$override = static function () {
+			return 'en_US';
+		};
+		add_filter( 'determine_locale', $override, 0 );
+
+		$defaults = $this->get_site_locale_defaults();
+
+		remove_filter( 'determine_locale', $override, 0 );
+		$locale = $old_locale;
+
+		$this->assertNull(
+			$defaults['date'],
+			'Date default must be null when the site locale pack is not installed.'
+		);
+		$this->assertNull(
+			$defaults['time'],
+			'Time default must be null when the site locale pack is not installed.'
+		);
+	}
+
+	/**
+	 * After the discovery block runs the switch path, no locale switch must
+	 * remain active. determine_locale() must return what it returned before.
+	 *
+	 * @ticket 64102
+	 * @covers ::switch_to_locale
+	 * @covers ::restore_previous_locale
+	 */
+	public function test_locale_is_fully_restored_after_discovery() {
+		global $wp_locale_switcher, $locale;
+		$old_locale = $locale;
+
+		$locale   = 'en_US'; // Site locale.
+		$override = static function () {
+			return 'de_DE';
+		};
+		add_filter( 'determine_locale', $override, 0 );
+
+		$before = determine_locale(); // 'de_DE' (from our filter).
+
+		$this->get_site_locale_defaults(); // Switches to en_US then restores.
+
+		$after = determine_locale(); // Must still be 'de_DE'.
+
+		remove_filter( 'determine_locale', $override, 0 );
+		$locale = $old_locale;
+
+		$this->assertSame(
+			$before,
+			$after,
+			'determine_locale() must return the same value before and after the discovery block.'
+		);
+		$this->assertFalse(
+			$wp_locale_switcher->is_switched(),
+			'No locale switch must remain active after the discovery block completes.'
+		);
+	}
+
+	// -------------------------------------------------------------------------
+	// Tests: label application logic
+	// -------------------------------------------------------------------------
+
+	/**
+	 * For a standard en_US site, exactly one entry in the date format list
+	 * should match the captured site locale default.
+	 *
+	 * @ticket 64102
+	 */
+	public function test_exactly_one_date_format_matches_site_locale_default() {
+		$defaults = $this->get_site_locale_defaults();
+
+		$date_formats = array_unique(
+			apply_filters( 'date_formats', array( __( 'F j, Y' ), 'Y-m-d', 'm/d/Y', 'd/m/Y', 'd.m.Y' ) )
+		);
+
+		$label_count = 0;
+		foreach ( $date_formats as $format ) {
+			if ( null !== $defaults['date'] && $format === $defaults['date'] ) {
+				++$label_count;
+			}
+		}
+
+		$this->assertSame(
+			1,
+			$label_count,
+			'Exactly one date format must match the site locale default so exactly one label is rendered.'
+		);
+	}
+
+	/**
+	 * For a standard en_US site, exactly one entry in the time format list
+	 * should match the captured site locale default.
+	 *
+	 * @ticket 64102
+	 */
+	public function test_exactly_one_time_format_matches_site_locale_default() {
+		$defaults = $this->get_site_locale_defaults();
+
+		$time_formats = array_unique(
+			apply_filters( 'time_formats', array( __( 'g:i a' ), 'g:i A', 'H:i' ) )
+		);
+
+		$label_count = 0;
+		foreach ( $time_formats as $format ) {
+			if ( null !== $defaults['time'] && $format === $defaults['time'] ) {
+				++$label_count;
+			}
+		}
+
+		$this->assertSame(
+			1,
+			$label_count,
+			'Exactly one time format must match the site locale default so exactly one label is rendered.'
+		);
+	}
+
+	/**
+	 * When the site locale pack is not installed, no format entry receives a label.
+	 *
+	 * @ticket 64102
+	 */
+	public function test_no_label_applied_when_site_locale_pack_not_installed() {
+		global $locale;
+		$old_locale = $locale;
+
+		$locale   = 'zz_ZZ'; // Site locale = non-existent.
+		$override = static function () {
+			return 'en_US';
+		};
+		add_filter( 'determine_locale', $override, 0 );
+
+		$defaults = $this->get_site_locale_defaults();
+
+		remove_filter( 'determine_locale', $override, 0 );
+		$locale = $old_locale;
+
+		$date_formats = array_unique(
+			apply_filters( 'date_formats', array( __( 'F j, Y' ), 'Y-m-d', 'm/d/Y', 'd/m/Y', 'd.m.Y' ) )
+		);
+
+		foreach ( $date_formats as $format ) {
+			$this->assertFalse(
+				null !== $defaults['date'] && $format === $defaults['date'],
+				'No date format must be labelled when the site locale pack is not installed.'
+			);
+		}
+	}
+
+	// -------------------------------------------------------------------------
+	// Tests: HTML structure
+	// -------------------------------------------------------------------------
+
+	/**
+	 * The "(Language default)" label is rendered as a sibling <span class="description">
+	 * and must NOT appear inside any <span class="date-time-text format-i18n"> element.
+	 * This protects the existing JS that reads .format-i18n text for preview updates.
+	 *
+	 * @ticket 64102
+	 * @see src/wp-admin/includes/options.php (jQuery click handler reading .format-i18n)
+	 */
+	public function test_label_is_never_inside_format_i18n_span() {
+		$defaults = $this->get_site_locale_defaults();
+
+		$date_formats = array_unique(
+			apply_filters( 'date_formats', array( __( 'F j, Y' ), 'Y-m-d', 'm/d/Y', 'd/m/Y', 'd.m.Y' ) )
+		);
+
+		ob_start();
+		foreach ( $date_formats as $format ) {
+			// Reproduce the exact HTML structure from options-general.php.
+			echo '<span class="date-time-text format-i18n">' . esc_html( date_i18n( $format ) ) . '</span>';
+			echo '<code>' . esc_html( $format ) . '</code>';
+			if ( null !== $defaults['date'] && $format === $defaults['date'] ) {
+				/* translators: Shown next to the date/time format that is the default for the site's language. */
+				echo ' <span class="description">' . esc_html__( '(Language default)' ) . '</span>';
+			}
+		}
+		$output = ob_get_clean();
+
+		// Assert: no .format-i18n span contains the label text.
+		preg_match_all(
+			'/<span class="date-time-text format-i18n">(.*?)<\/span>/s',
+			$output,
+			$matches
+		);
+
+		foreach ( $matches[1] as $span_content ) {
+			$this->assertStringNotContainsString(
+				'Language default',
+				$span_content,
+				'The "(Language default)" label must not appear inside a .format-i18n span.'
+			);
+		}
+	}
+}