diff --git a/src/wp-includes/date.php b/src/wp-includes/date.php
index ea2b4a60a2..0a789337ad 100644
--- a/src/wp-includes/date.php
+++ b/src/wp-includes/date.php
@@ -815,16 +815,59 @@ class WP_Date_Query {
 	 * @since 3.7.0
 	 * @access public
 	 *
-	 * @param string|array $datetime An array of parameters or a strotime() string
-	 * @param string $default_to Controls what values default to if they are missing from $datetime. Pass "min" or "max".
+	 * @param string|array $datetime       An array of parameters or a strotime() string
+	 * @param bool         $default_to_max Whether to round up incomplete dates. Supported by values
+	 *                                     of $datetime that are arrays, or string values that are a
+	 *                                     subset of MySQL date format ('Y', 'Y-m', 'Y-m-d', 'Y-m-d H:i').
+	 *                                     Default: false.
 	 * @return string|false A MySQL format date/time or false on failure
 	 */
 	public function build_mysql_datetime( $datetime, $default_to_max = false ) {
 		$now = current_time( 'timestamp' );
 
 		if ( ! is_array( $datetime ) ) {
-			// @todo Timezone issues here possibly
-			return gmdate( 'Y-m-d H:i:s', strtotime( $datetime, $now ) );
+
+			/*
+			 * Try to parse some common date formats, so we can detect
+			 * the level of precision and support the 'inclusive' parameter.
+			 */
+			if ( preg_match( '/^(\d{4})$/', $datetime, $matches ) ) {
+				// Y
+				$datetime = array(
+					'year' => intval( $matches[1] ),
+				);
+
+			} else if ( preg_match( '/^(\d{4})\-(\d{2})$/', $datetime, $matches ) ) {
+				// Y-m
+				$datetime = array(
+					'year'  => intval( $matches[1] ),
+					'month' => intval( $matches[2] ),
+				);
+
+			} else if ( preg_match( '/^(\d{4})\-(\d{2})\-(\d{2})$/', $datetime, $matches ) ) {
+				// Y-m-d
+				$datetime = array(
+					'year'  => intval( $matches[1] ),
+					'month' => intval( $matches[2] ),
+					'day'   => intval( $matches[3] ),
+				);
+
+			} else if ( preg_match( '/^(\d{4})\-(\d{2})\-(\d{2}) (\d{2}):(\d{2})$/', $datetime, $matches ) ) {
+				// Y-m-d H:i
+				$datetime = array(
+					'year'   => intval( $matches[1] ),
+					'month'  => intval( $matches[2] ),
+					'day'    => intval( $matches[3] ),
+					'hour'   => intval( $matches[4] ),
+					'minute' => intval( $matches[5] ),
+				);
+			}
+
+			// If no match is found, we don't support default_to_max.
+			if ( ! is_array( $datetime ) ) {
+				// @todo Timezone issues here possibly
+				return gmdate( 'Y-m-d H:i:s', strtotime( $datetime, $now ) );
+			}
 		}
 
 		$datetime = array_map( 'absint', $datetime );
diff --git a/tests/phpunit/tests/query/dateQuery.php b/tests/phpunit/tests/query/dateQuery.php
index 676d860ce9..7570d7b8ac 100644
--- a/tests/phpunit/tests/query/dateQuery.php
+++ b/tests/phpunit/tests/query/dateQuery.php
@@ -271,6 +271,378 @@ class Tests_Query_DateQuery extends WP_UnitTestCase {
 		$this->assertEquals( $expected_dates, wp_list_pluck( $posts, 'post_date' ) );
 	}
 
+	/**
+	 * @ticket 29908
+	 */
+	public function test_beforeafter_with_date_string_Y() {
+		$p1 = $this->factory->post->create( array(
+			'post_date' => '2008-05-06 13:00:00',
+		) );
+		$p2 = $this->factory->post->create( array(
+			'post_date' => '2007-05-07 13:00:00',
+		) );
+
+		$before_posts = $this->_get_query_result( array(
+			'fields' => 'ids',
+			'update_post_meta_cache' => false,
+			'update_post_term_cache' => false,
+			'date_query' => array(
+				'before' => '2008',
+			),
+		) );
+
+		$after_posts = $this->_get_query_result( array(
+			'fields' => 'ids',
+			'update_post_meta_cache' => false,
+			'update_post_term_cache' => false,
+			'date_query' => array(
+				'after' => '2007',
+			),
+		) );
+
+		$this->assertEquals( array( $p2 ), $before_posts );
+		$this->assertEquals( array( $p1 ), $after_posts );
+	}
+
+	/**
+	 * @ticket 29908
+	 */
+	public function test_beforeafter_with_date_string_Y_inclusive() {
+		$p1 = $this->factory->post->create( array(
+			'post_date' => '2008-05-06 13:00:00',
+		) );
+		$p2 = $this->factory->post->create( array(
+			'post_date' => '2007-05-07 13:00:00',
+		) );
+
+		$before_posts = $this->_get_query_result( array(
+			'fields' => 'ids',
+			'update_post_meta_cache' => false,
+			'update_post_term_cache' => false,
+			'date_query' => array(
+				'before' => '2008',
+				'inclusive' => true,
+			),
+		) );
+
+		$after_posts = $this->_get_query_result( array(
+			'fields' => 'ids',
+			'update_post_meta_cache' => false,
+			'update_post_term_cache' => false,
+			'date_query' => array(
+				'after' => '2007',
+				'inclusive' => true,
+			),
+		) );
+
+		$this->assertEqualSets( array( $p1, $p2 ), $before_posts );
+		$this->assertEqualSets( array( $p1, $p2 ), $after_posts );
+	}
+
+	/**
+	 * @ticket 29908
+	 */
+	public function test_beforeafter_with_date_string_Ym() {
+		$p1 = $this->factory->post->create( array(
+			'post_date' => '2008-05-06 13:00:00',
+		) );
+		$p2 = $this->factory->post->create( array(
+			'post_date' => '2008-04-07 13:00:00',
+		) );
+
+		$before_posts = $this->_get_query_result( array(
+			'fields' => 'ids',
+			'update_post_meta_cache' => false,
+			'update_post_term_cache' => false,
+			'date_query' => array(
+				'before' => '2008-05',
+			),
+		) );
+
+		$after_posts = $this->_get_query_result( array(
+			'fields' => 'ids',
+			'update_post_meta_cache' => false,
+			'update_post_term_cache' => false,
+			'date_query' => array(
+				'after' => '2008-04',
+			),
+		) );
+
+		$this->assertEquals( array( $p2 ), $before_posts );
+		$this->assertEquals( array( $p1 ), $after_posts );
+	}
+
+	/**
+	 * @ticket 29908
+	 */
+	public function test_beforeafter_with_date_string_Ym_inclusive() {
+		$p1 = $this->factory->post->create( array(
+			'post_date' => '2008-05-06 13:00:00',
+		) );
+		$p2 = $this->factory->post->create( array(
+			'post_date' => '2008-04-07 13:00:00',
+		) );
+
+		$before_posts = $this->_get_query_result( array(
+			'fields' => 'ids',
+			'update_post_meta_cache' => false,
+			'update_post_term_cache' => false,
+			'date_query' => array(
+				'before' => '2008-05',
+				'inclusive' => true,
+			),
+		) );
+
+		$after_posts = $this->_get_query_result( array(
+			'fields' => 'ids',
+			'update_post_meta_cache' => false,
+			'update_post_term_cache' => false,
+			'date_query' => array(
+				'after' => '2008-04',
+				'inclusive' => true,
+			),
+		) );
+
+		$this->assertEqualSets( array( $p1, $p2 ), $before_posts );
+		$this->assertEqualSets( array( $p1, $p2 ), $after_posts );
+	}
+
+	/**
+	 * @ticket 29908
+	 */
+	public function test_beforeafter_with_date_string_Ymd() {
+		$p1 = $this->factory->post->create( array(
+			'post_date' => '2008-05-06 13:00:00',
+		) );
+		$p2 = $this->factory->post->create( array(
+			'post_date' => '2008-05-05 13:00:00',
+		) );
+
+		$before_posts = $this->_get_query_result( array(
+			'fields' => 'ids',
+			'update_post_meta_cache' => false,
+			'update_post_term_cache' => false,
+			'date_query' => array(
+				'before' => '2008-05-06',
+			),
+		) );
+
+		$after_posts = $this->_get_query_result( array(
+			'fields' => 'ids',
+			'update_post_meta_cache' => false,
+			'update_post_term_cache' => false,
+			'date_query' => array(
+				'after' => '2008-05-05',
+			),
+		) );
+
+		$this->assertEquals( array( $p2 ), $before_posts );
+		$this->assertEquals( array( $p1 ), $after_posts );
+	}
+
+	/**
+	 * @ticket 29908
+	 */
+	public function test_beforeafter_with_date_string_Ymd_inclusive() {
+		$p1 = $this->factory->post->create( array(
+			'post_date' => '2008-05-06 13:00:00',
+		) );
+		$p2 = $this->factory->post->create( array(
+			'post_date' => '2008-05-05 13:00:00',
+		) );
+
+		$before_posts = $this->_get_query_result( array(
+			'fields' => 'ids',
+			'update_post_meta_cache' => false,
+			'update_post_term_cache' => false,
+			'date_query' => array(
+				'before' => '2008-05-06',
+				'inclusive' => true,
+			),
+		) );
+
+		$after_posts = $this->_get_query_result( array(
+			'fields' => 'ids',
+			'update_post_meta_cache' => false,
+			'update_post_term_cache' => false,
+			'date_query' => array(
+				'after' => '2008-05-05',
+				'inclusive' => true,
+			),
+		) );
+
+		$this->assertEqualSets( array( $p1, $p2 ), $before_posts );
+		$this->assertEqualSets( array( $p1, $p2 ), $after_posts );
+	}
+
+	/**
+	 * @ticket 29908
+	 */
+	public function test_beforeafter_with_date_string_YmdHi() {
+		$p1 = $this->factory->post->create( array(
+			'post_date' => '2008-05-06 14:05:00',
+		) );
+		$p2 = $this->factory->post->create( array(
+			'post_date' => '2008-05-06 14:04:00',
+		) );
+
+		$before_posts = $this->_get_query_result( array(
+			'fields' => 'ids',
+			'update_post_meta_cache' => false,
+			'update_post_term_cache' => false,
+			'date_query' => array(
+				'before' => '2008-05-06 14:05',
+			),
+		) );
+
+		$after_posts = $this->_get_query_result( array(
+			'fields' => 'ids',
+			'update_post_meta_cache' => false,
+			'update_post_term_cache' => false,
+			'date_query' => array(
+				'after' => '2008-05-06 14:04',
+			),
+		) );
+
+		$this->assertEquals( array( $p2 ), $before_posts );
+		$this->assertEquals( array( $p1 ), $after_posts );
+	}
+
+	/**
+	 * @ticket 29908
+	 */
+	public function test_beforeafter_with_date_string_YmdHi_inclusive() {
+		$p1 = $this->factory->post->create( array(
+			'post_date' => '2008-05-06 14:05:00',
+		) );
+		$p2 = $this->factory->post->create( array(
+			'post_date' => '2008-05-06 14:04:00',
+		) );
+
+		$before_posts = $this->_get_query_result( array(
+			'fields' => 'ids',
+			'update_post_meta_cache' => false,
+			'update_post_term_cache' => false,
+			'date_query' => array(
+				'before' => '2008-05-06 14:05',
+				'inclusive' => true,
+			),
+		) );
+
+		$after_posts = $this->_get_query_result( array(
+			'fields' => 'ids',
+			'update_post_meta_cache' => false,
+			'update_post_term_cache' => false,
+			'date_query' => array(
+				'after' => '2008-05-06 14:04',
+				'inclusive' => true,
+			),
+		) );
+
+		$this->assertEqualSets( array( $p1, $p2 ), $before_posts );
+		$this->assertEqualSets( array( $p1, $p2 ), $after_posts );
+	}
+
+	/**
+	 * @ticket 29908
+	 */
+	public function test_beforeafter_with_date_string_YmdHis() {
+		$p1 = $this->factory->post->create( array(
+			'post_date' => '2008-05-06 14:05:15',
+		) );
+		$p2 = $this->factory->post->create( array(
+			'post_date' => '2008-05-06 14:05:14',
+		) );
+
+		$before_posts = $this->_get_query_result( array(
+			'fields' => 'ids',
+			'update_post_meta_cache' => false,
+			'update_post_term_cache' => false,
+			'date_query' => array(
+				'before' => '2008-05-06 14:05:15',
+			),
+		) );
+
+		$after_posts = $this->_get_query_result( array(
+			'fields' => 'ids',
+			'update_post_meta_cache' => false,
+			'update_post_term_cache' => false,
+			'date_query' => array(
+				'after' => '2008-05-06 14:05:14',
+			),
+		) );
+
+		$this->assertEquals( array( $p2 ), $before_posts );
+		$this->assertEquals( array( $p1 ), $after_posts );
+	}
+
+	/**
+	 * @ticket 29908
+	 */
+	public function test_beforeafter_with_date_string_YmdHis_inclusive() {
+		$p1 = $this->factory->post->create( array(
+			'post_date' => '2008-05-06 14:04:15',
+		) );
+		$p2 = $this->factory->post->create( array(
+			'post_date' => '2008-05-06 14:04:14',
+		) );
+
+		$before_posts = $this->_get_query_result( array(
+			'fields' => 'ids',
+			'update_post_meta_cache' => false,
+			'update_post_term_cache' => false,
+			'date_query' => array(
+				'before' => '2008-05-06 14:04:15',
+				'inclusive' => true,
+			),
+		) );
+
+		$after_posts = $this->_get_query_result( array(
+			'fields' => 'ids',
+			'update_post_meta_cache' => false,
+			'update_post_term_cache' => false,
+			'date_query' => array(
+				'after' => '2008-05-06 14:04:14',
+				'inclusive' => true,
+			),
+		) );
+
+		$this->assertEqualSets( array( $p1, $p2 ), $before_posts );
+		$this->assertEqualSets( array( $p1, $p2 ), $after_posts );
+	}
+
+	/**
+	 * @ticket 29908
+	 */
+	public function test_beforeafter_with_date_string_non_parseable() {
+		$p1 = $this->factory->post->create( array(
+			'post_date' => '2008-05-06 14:05:15',
+		) );
+		$p2 = $this->factory->post->create( array(
+			'post_date' => '2008-05-06 14:05:14',
+		) );
+
+		$before_posts = $this->_get_query_result( array(
+			'fields' => 'ids',
+			'update_post_meta_cache' => false,
+			'update_post_term_cache' => false,
+			'date_query' => array(
+				'before' => 'June 12, 2008',
+			),
+		) );
+
+		$after_posts = $this->_get_query_result( array(
+			'fields' => 'ids',
+			'update_post_meta_cache' => false,
+			'update_post_term_cache' => false,
+			'date_query' => array(
+				'after' => 'June 12, 2007',
+			),
+		) );
+
+		$this->assertEquals( array( $p1, $p2 ), $before_posts );
+	}
+
 	public function test_date_query_year_expecting_results() {
 		$this->create_posts();