diff --git src/wp-includes/post.php src/wp-includes/post.php
index 564d4ee..b0b1391 100644
--- src/wp-includes/post.php
+++ src/wp-includes/post.php
@@ -3770,6 +3770,27 @@ function wp_unique_post_slug( $slug, $post_ID, $post_status, $post_type, $post_p
 		$check_sql = "SELECT post_name FROM $wpdb->posts WHERE post_name = %s AND post_type = %s AND ID != %d LIMIT 1";
 		$post_name_check = $wpdb->get_var( $wpdb->prepare( $check_sql, $slug, $post_type, $post_ID ) );
 
+		// Prevent post slugs that could result in URLs that conflict with date archives.
+		$conflicts_with_date_archive = false;
+		if ( 'post' === $post_type && preg_match( '/^[0-9]+$/', $slug ) && $slug_num = intval( $slug ) ) {
+			$permastructs   = array_values( array_filter( explode( '/', get_option( 'permalink_structure' ) ) ) );
+			$postname_index = array_search( '%postname%', $permastructs );
+
+			/*
+			 * Potential date clashes are as follows:
+			 *
+			 * - Any integer in the first permastruct position could be a year.
+			 * - An integer between 1 and 12 that follows 'year' conflicts with 'monthnum'.
+			 * - An integer between 1 and 31 that follows 'monthnum' conflicts with 'day'.
+			 */
+			if ( 0 === $postname_index ||
+				( $postname_index && '%year%' === $permastructs[ $postname_index - 1 ] && 13 > $slug_num ) ||
+				( $postname_index && '%monthnum%' === $permastructs[ $postname_index - 1 ] && 32 > $slug_num )
+			) {
+				$conflicts_with_date_archive = true;
+			}
+		}
+
 		/**
 		 * Filter whether the post slug would be bad as a flat slug.
 		 *
@@ -3779,7 +3800,7 @@ function wp_unique_post_slug( $slug, $post_ID, $post_status, $post_type, $post_p
 		 * @param string $slug      The post slug.
 		 * @param string $post_type Post type.
 		 */
-		if ( $post_name_check || in_array( $slug, $feeds ) || apply_filters( 'wp_unique_post_slug_is_bad_flat_slug', false, $slug, $post_type ) ) {
+		if ( $post_name_check || in_array( $slug, $feeds ) || $conflicts_with_date_archive || apply_filters( 'wp_unique_post_slug_is_bad_flat_slug', false, $slug, $post_type ) ) {
 			$suffix = 2;
 			do {
 				$alt_post_name = _truncate_post_slug( $slug, 200 - ( strlen( $suffix ) + 1 ) ) . "-$suffix";
diff --git tests/phpunit/tests/admin/includesPost.php tests/phpunit/tests/admin/includesPost.php
index 5d2623a..e25a917 100644
--- tests/phpunit/tests/admin/includesPost.php
+++ tests/phpunit/tests/admin/includesPost.php
@@ -313,4 +313,168 @@ class Tests_Admin_includesPost extends WP_UnitTestCase {
 		$wp_rewrite->set_permalink_structure( $old_permalink_structure );
 		flush_rewrite_rules();
 	}
+
+	/**
+	 * @ticket 5305
+	 */
+	public function test_get_sample_permalink_should_avoid_slugs_that_would_create_clashes_with_year_archives() {
+		global $wp_rewrite;
+		$wp_rewrite->init();
+		$wp_rewrite->set_permalink_structure( '/%postname%/' );
+		$wp_rewrite->flush_rules();
+
+		$p = $this->factory->post->create( array(
+			'post_name' => '2015',
+		) );
+
+		$found = get_sample_permalink( $p );
+		$this->assertEquals( '2015-2', $found[1] );
+
+		$wp_rewrite->set_permalink_structure( '' );
+		flush_rewrite_rules();
+	}
+
+	/**
+	 * @ticket 5305
+	 */
+	public function test_get_sample_permalink_should_allow_yearlike_slugs_if_permastruct_does_not_cause_an_archive_conflict() {
+		global $wp_rewrite;
+		$wp_rewrite->init();
+		$wp_rewrite->set_permalink_structure( '/%year%/%postname%/' );
+		$wp_rewrite->flush_rules();
+
+		$p = $this->factory->post->create( array(
+			'post_name' => '2015',
+		) );
+
+		$found = get_sample_permalink( $p );
+		$this->assertEquals( '2015', $found[1] );
+
+		$wp_rewrite->set_permalink_structure( '' );
+		flush_rewrite_rules();
+	}
+
+	/**
+	 * @ticket 5305
+	 */
+	public function test_get_sample_permalink_should_avoid_slugs_that_would_create_clashes_with_month_archives() {
+		global $wp_rewrite;
+		$wp_rewrite->init();
+		$wp_rewrite->set_permalink_structure( '/%year%/%postname%/' );
+		$wp_rewrite->flush_rules();
+
+		$p = $this->factory->post->create( array(
+			'post_name' => '11',
+		) );
+
+		$found = get_sample_permalink( $p );
+		$this->assertEquals( '11-2', $found[1] );
+
+		$wp_rewrite->set_permalink_structure( '' );
+		flush_rewrite_rules();
+	}
+
+	/**
+	 * @ticket 5305
+	 */
+	public function test_get_sample_permalink_should_ignore_potential_month_conflicts_for_invalid_monthnum() {
+		global $wp_rewrite;
+		$wp_rewrite->init();
+		$wp_rewrite->set_permalink_structure( '/%year%/%postname%/' );
+		$wp_rewrite->flush_rules();
+
+		$p = $this->factory->post->create( array(
+			'post_name' => '13',
+		) );
+
+		$found = get_sample_permalink( $p );
+		$this->assertEquals( '13', $found[1] );
+
+		$wp_rewrite->set_permalink_structure( '' );
+		flush_rewrite_rules();
+	}
+
+	/**
+	 * @ticket 5305
+	 */
+	public function test_get_sample_permalink_should_avoid_slugs_that_would_create_clashes_with_day_archives() {
+		global $wp_rewrite;
+		$wp_rewrite->init();
+		$wp_rewrite->set_permalink_structure( '/%year%/%monthnum%/%postname%/' );
+		$wp_rewrite->flush_rules();
+
+		$p = $this->factory->post->create( array(
+			'post_name' => '30',
+		) );
+
+		$found = get_sample_permalink( $p );
+		$this->assertEquals( '30-2', $found[1] );
+
+		$wp_rewrite->set_permalink_structure( '' );
+		flush_rewrite_rules();
+	}
+
+	/**
+	 * @ticket 5305
+	 */
+	public function test_get_sample_permalink_should_iterate_slug_suffix_when_a_date_conflict_is_found() {
+		global $wp_rewrite;
+		$wp_rewrite->init();
+		$wp_rewrite->set_permalink_structure( '/%year%/%monthnum%/%postname%/' );
+		$wp_rewrite->flush_rules();
+
+		$this->factory->post->create( array(
+			'post_name' => '30-2',
+		) );
+
+		$p = $this->factory->post->create( array(
+			'post_name' => '30',
+		) );
+
+		$found = get_sample_permalink( $p );
+		$this->assertEquals( '30-3', $found[1] );
+
+		$wp_rewrite->set_permalink_structure( '' );
+		flush_rewrite_rules();
+	}
+
+	/**
+	 * @ticket 5305
+	 */
+	public function test_get_sample_permalink_should_ignore_potential_day_conflicts_for_invalid_day() {
+		global $wp_rewrite;
+		$wp_rewrite->init();
+		$wp_rewrite->set_permalink_structure( '/%year%/%monthnum%/%postname%/' );
+		$wp_rewrite->flush_rules();
+
+		$p = $this->factory->post->create( array(
+			'post_name' => '32',
+		) );
+
+		$found = get_sample_permalink( $p );
+		$this->assertEquals( '32', $found[1] );
+
+		$wp_rewrite->set_permalink_structure( '' );
+		flush_rewrite_rules();
+	}
+
+	/**
+	 * @ticket 5305
+	 */
+	public function test_get_sample_permalink_should_allow_daylike_slugs_if_permastruct_does_not_cause_an_archive_conflict() {
+		global $wp_rewrite;
+		$wp_rewrite->init();
+		$wp_rewrite->set_permalink_structure( '/%year%/%month%/%day%/%postname%/' );
+		$wp_rewrite->flush_rules();
+
+		$p = $this->factory->post->create( array(
+			'post_name' => '30',
+		) );
+
+		$found = get_sample_permalink( $p );
+		$this->assertEquals( '30', $found[1] );
+
+		$wp_rewrite->set_permalink_structure( '' );
+		flush_rewrite_rules();
+	}
 }
diff --git tests/phpunit/tests/post/wpUniquePostSlug.php tests/phpunit/tests/post/wpUniquePostSlug.php
index 019e6bd..3256172 100644
--- tests/phpunit/tests/post/wpUniquePostSlug.php
+++ tests/phpunit/tests/post/wpUniquePostSlug.php
@@ -164,4 +164,172 @@ class Tests_Post_WpUniquePostSlug extends WP_UnitTestCase {
 
 		$this->assertSame( 'foo', $actual );
 	}
+
+	/**
+	 * @ticket 5305
+	 */
+	public function test_slugs_resulting_in_permalinks_that_resemble_year_archives_should_be_suffixed() {
+		global $wp_rewrite;
+		$wp_rewrite->init();
+		$wp_rewrite->set_permalink_structure( '/%postname%/' );
+		$wp_rewrite->flush_rules();
+
+		$p = $this->factory->post->create( array(
+			'post_type' => 'post',
+			'post_name' => 'foo',
+		) );
+
+		$found = wp_unique_post_slug( '2015', $p, 'publish', 'post', 0 );
+		$this->assertEquals( '2015-2', $found );
+
+		$wp_rewrite->set_permalink_structure( '' );
+		flush_rewrite_rules();
+	}
+
+	/**
+	 * @ticket 5305
+	 */
+	public function test_yearlike_slugs_should_not_be_suffixed_if_permalink_structure_does_not_result_in_a_clash_with_year_archives() {
+		global $wp_rewrite;
+		$wp_rewrite->init();
+		$wp_rewrite->set_permalink_structure( '/%year%/%postname%/' );
+		$wp_rewrite->flush_rules();
+
+		$p = $this->factory->post->create( array(
+			'post_type' => 'post',
+			'post_name' => 'foo',
+		) );
+
+		$found = wp_unique_post_slug( '2015', $p, 'publish', 'post', 0 );
+		$this->assertEquals( '2015', $found );
+
+		$wp_rewrite->set_permalink_structure( '' );
+		flush_rewrite_rules();
+	}
+
+	/**
+	 * @ticket 5305
+	 */
+	public function test_slugs_resulting_in_permalinks_that_resemble_month_archives_should_be_suffixed() {
+		global $wp_rewrite;
+		$wp_rewrite->init();
+		$wp_rewrite->set_permalink_structure( '/%year%/%postname%/' );
+		$wp_rewrite->flush_rules();
+
+		$p = $this->factory->post->create( array(
+			'post_type' => 'post',
+			'post_name' => 'foo',
+		) );
+
+		$found = wp_unique_post_slug( '11', $p, 'publish', 'post', 0 );
+		$this->assertEquals( '11-2', $found );
+
+		$wp_rewrite->set_permalink_structure( '' );
+		flush_rewrite_rules();
+	}
+
+	/**
+	 * @ticket 5305
+	 */
+	public function test_monthlike_slugs_should_not_be_suffixed_if_permalink_structure_does_not_result_in_a_clash_with_month_archives() {
+		global $wp_rewrite;
+		$wp_rewrite->init();
+		$wp_rewrite->set_permalink_structure( '/%year%/foo/%postname%/' );
+		$wp_rewrite->flush_rules();
+
+		$p = $this->factory->post->create( array(
+			'post_type' => 'post',
+			'post_name' => 'foo',
+		) );
+
+		$found = wp_unique_post_slug( '11', $p, 'publish', 'post', 0 );
+		$this->assertEquals( '11', $found );
+
+		$wp_rewrite->set_permalink_structure( '' );
+		flush_rewrite_rules();
+	}
+
+	/**
+	 * @ticket 5305
+	 */
+	public function test_monthlike_slugs_should_not_be_suffixed_for_invalid_month_numbers() {
+		global $wp_rewrite;
+		$wp_rewrite->init();
+		$wp_rewrite->set_permalink_structure( '/%year%/%postname%/' );
+		$wp_rewrite->flush_rules();
+
+		$p = $this->factory->post->create( array(
+			'post_type' => 'post',
+			'post_name' => 'foo',
+		) );
+
+		$found = wp_unique_post_slug( '13', $p, 'publish', 'post', 0 );
+		$this->assertEquals( '13', $found );
+
+		$wp_rewrite->set_permalink_structure( '' );
+		flush_rewrite_rules();
+	}
+
+	/**
+	 * @ticket 5305
+	 */
+	public function test_slugs_resulting_in_permalinks_that_resemble_day_archives_should_be_suffixed() {
+		global $wp_rewrite;
+		$wp_rewrite->init();
+		$wp_rewrite->set_permalink_structure( '/%year%/%monthnum%/%postname%/' );
+		$wp_rewrite->flush_rules();
+
+		$p = $this->factory->post->create( array(
+			'post_type' => 'post',
+			'post_name' => 'foo',
+		) );
+
+		$found = wp_unique_post_slug( '30', $p, 'publish', 'post', 0 );
+		$this->assertEquals( '30-2', $found );
+
+		$wp_rewrite->set_permalink_structure( '' );
+		flush_rewrite_rules();
+	}
+
+	/**
+	 * @ticket 5305
+	 */
+	public function test_daylike_slugs_should_not_be_suffixed_if_permalink_structure_does_not_result_in_a_clash_with_day_archives() {
+		global $wp_rewrite;
+		$wp_rewrite->init();
+		$wp_rewrite->set_permalink_structure( '/%year%/%monthnum%/%day%/%postname%/' );
+		$wp_rewrite->flush_rules();
+
+		$p = $this->factory->post->create( array(
+			'post_type' => 'post',
+			'post_name' => 'foo',
+		) );
+
+		$found = wp_unique_post_slug( '30', $p, 'publish', 'post', 0 );
+		$this->assertEquals( '30', $found );
+
+		$wp_rewrite->set_permalink_structure( '' );
+		flush_rewrite_rules();
+	}
+
+	/**
+	 * @ticket 5305
+	 */
+	public function test_daylike_slugs_should_not_be_suffixed_for_invalid_day_numbers() {
+		global $wp_rewrite;
+		$wp_rewrite->init();
+		$wp_rewrite->set_permalink_structure( '/%year%/%monthnum%/%postname%/' );
+		$wp_rewrite->flush_rules();
+
+		$p = $this->factory->post->create( array(
+			'post_type' => 'post',
+			'post_name' => 'foo',
+		) );
+
+		$found = wp_unique_post_slug( '32', $p, 'publish', 'post', 0 );
+		$this->assertEquals( '32', $found );
+
+		$wp_rewrite->set_permalink_structure( '' );
+		flush_rewrite_rules();
+	}
 }