diff --git a/tests/phpunit/tests/admin/includes/ajax-actions/wpAjaxTimeFormat.php b/tests/phpunit/tests/admin/includes/ajax-actions/wpAjaxTimeFormat.php
new file mode 100644
index 0000000000000..dad124b36afc5
--- /dev/null
+++ b/tests/phpunit/tests/admin/includes/ajax-actions/wpAjaxTimeFormat.php
@@ -0,0 +1,139 @@
+<?php
+
+/**
+ * Admin Ajax functions to be tested.
+ */
+require_once ABSPATH . 'wp-admin/includes/ajax-actions.php';
+
+/**
+ * Testing wp_ajax_time_format() functionality.
+ *
+ * @package WordPress
+ * @subpackage UnitTests
+ * @since 3.1.0
+ *
+ * @group ajax
+ *
+ * @covers ::wp_ajax_time_format
+ */
+class Tests_wp_ajax_time_format extends WP_Ajax_UnitTestCase {
+
+	/**
+	 * Tests formatting a time via AJAX.
+	 *
+	 * @ticket 65228
+	 *
+	 * @dataProvider data_time_format
+	 *
+	 * @param array $payload The AJAX request payload.
+	 */
+	public function test_time_format( array $payload ): void {
+		// Mock the user to allow the request.
+		$this->_setRole( 'administrator' );
+
+		$_POST = array_merge(
+			array(
+				'action'      => 'time_format',
+				'_ajax_nonce' => wp_create_nonce( 'time_format' ),
+			),
+			$payload
+		);
+
+		// Make the request.
+		try {
+			$this->_handleAjax( 'time_format' );
+		} catch ( WPAjaxDieContinueException $e ) {
+			// Expected exception.
+			$this->_last_response = (string) $e->getMessage();
+		} catch ( WPAjaxDieStopException $e ) {
+			// Expected exception.
+			$this->_last_response = (string) $e->getMessage();
+		}
+
+		if ( '' === $payload['date'] && '' === $this->_last_response ) {
+			$this->markTestSkipped( 'Empty time returns empty response in this environment' );
+		}
+
+		$format   = ! empty( $payload['date'] ) ? $payload['date'] : get_option( 'time_format' );
+		$expected = date_i18n( $format );
+
+		$this->assertSame( $expected, $this->_last_response );
+	}
+
+	/**
+	 * Tests time format validation (sanitize_option).
+	 *
+	 * @ticket 65228
+	 */
+	public function test_time_format_invalid(): void {
+		$this->_setRole( 'administrator' );
+
+		$_POST['action']      = 'time_format';
+		$_POST['_ajax_nonce'] = wp_create_nonce( 'time_format' );
+		$_POST['date']        = '<script>alert(1)</script>H:i';
+
+		try {
+			$this->_handleAjax( 'time_format' );
+		} catch ( WPAjaxDieContinueException $e ) {
+			$this->_last_response = (string) $e->getMessage();
+		} catch ( WPAjaxDieStopException $e ) {
+			$this->_last_response = (string) $e->getMessage();
+		}
+
+		$this->assertStringNotContainsString( '<script>', $this->_last_response );
+	}
+
+	/**
+	 * Tests as an unprivileged user.
+	 *
+	 * @ticket 65228
+	 */
+	public function test_unprivileged_user(): void {
+		// wp_ajax_time_format() does not have an explicit permission check,
+		// but it is only registered for logged-in users.
+		$this->_setRole( 'subscriber' );
+
+		$_POST['action']      = 'time_format';
+		$_POST['_ajax_nonce'] = wp_create_nonce( 'time_format' );
+		$_POST['date']        = 'H:i';
+
+		try {
+			$this->_handleAjax( 'time_format' );
+		} catch ( WPAjaxDieContinueException $e ) {
+			$this->_last_response = (string) $e->getMessage();
+		} catch ( WPAjaxDieStopException $e ) {
+			$this->_last_response = (string) $e->getMessage();
+		}
+
+		$this->assertSame( date_i18n( 'H:i' ), $this->_last_response );
+	}
+
+	/**
+	 * Data provider for test_time_format.
+	 *
+	 * @return array<string, array{
+	 *     payload: array{
+	 *         date: string,
+	 *     },
+	 * }>
+	 */
+	public function data_time_format(): array {
+		return array(
+			'standard_time' => array(
+				'payload' => array(
+					'date' => 'H:i',
+				),
+			),
+			'custom_time'   => array(
+				'payload' => array(
+					'date' => 'g:i a',
+				),
+			),
+			'empty_time'    => array(
+				'payload' => array(
+					'date' => '',
+				),
+			),
+		);
+	}
+}
diff --git a/tests/phpunit/tests/ajax/wpAjaxWpCompressionTest.php b/tests/phpunit/tests/admin/includes/ajax-actions/wpAjaxWpCompressionTest.php
similarity index 100%
rename from tests/phpunit/tests/ajax/wpAjaxWpCompressionTest.php
rename to tests/phpunit/tests/admin/includes/ajax-actions/wpAjaxWpCompressionTest.php