Updated version of get upcoming birthdays, within an interval, such as month or week (default), based on gist by @rythie. Two types of queries (pick which you want), one uses separate meta data for DOB (day and month used), the other uses the DOB as a timestamp. Optional parameter for order.

birthdays.php

<?php
/**
 * get_birthdays
 * @desc	Get upcoming birthdays, within a month or week (default)
 * @see		http://dev.mysql.com/doc/refman/5.5/en/date-and-time-functions.html
 * @global	object		$wpdb
 * @param	string|int	$type
 * @param	string		$orderby	(asc or desc)
 * @return	array
 */
public static function get_birthdays($type = 'week', $orderby = 'ASC') {
	global $wpdb;

	switch($type) {
		case 'month':
			$interval = 30;
			break;

		case 'week':
		default:
			$interval = 7;
			break;
	}

	if(is_int($type)) {
		$interval = $type;
	}

	$order_by = in_array(strtoupper($orderby), array('ASC', 'DESC')) ? $orderby : 'ASC';

	// uses separate user meta data; day and month
	$sql = 'SELECT user.*
			FROM %1$s AS user
			LEFT JOIN wp_usermeta as meta_month ON user.ID = meta_month.user_id AND meta_month.meta_key = "date_dob_month"
			LEFT JOIN wp_usermeta as meta_day ON user.ID = meta_day.user_id AND meta_day.meta_key = "date_dob_day"
			WHERE user.user_status = 0
			AND DATEDIFF(
				DATE(
					CONCAT(
						YEAR(DATE_ADD(NOW(), INTERVAL %2$d DAY)), "-",
						meta_month.meta_value, "-" ,
						meta_day.meta_value
					)
				),
				DATE(NOW())
			) BETWEEN 0 AND %2$d
			ORDER BY UNIX_TIMESTAMP(
				CONCAT(
					YEAR(DATE_ADD(NOW(), INTERVAL %2$d DAY)), "-",
					meta_month.meta_value, "-" ,
					meta_day.meta_value
				)
			) %3$s';

	// based on the user dob stored as unix timestamp
	// because of potential negative epoch, use the follow http://www.epochconverter.com/programming/mysql-from-unixtime.php#negavtiveEpoch
	$sql = 'SELECT user.*
			FROM %1$s AS user
			LEFT JOIN wp_usermeta as meta ON user.ID = meta.user_id AND meta.meta_key = "date_dob"
			WHERE user.user_status = 0
			AND meta.meta_value != ""
			AND DATEDIFF(
				DATE(
					CONCAT(
						YEAR(DATE_ADD(NOW(), INTERVAL %2$d DAY)), "-",
						MONTH(DATE(DATE_ADD(FROM_UNIXTIME(0), INTERVAL meta.meta_value SECOND))), "-" ,
						DAY(DATE(DATE_ADD(FROM_UNIXTIME(0), INTERVAL meta.meta_value SECOND)))
					)
				),
				DATE(NOW())
			) BETWEEN 0 AND %2$d
			ORDER BY UNIX_TIMESTAMP(DATE_ADD(FROM_UNIXTIME(0), INTERVAL meta.meta_value SECOND)) %3$s';

	$query = sprintf($sql, $wpdb->users, $interval, $order_by);

	return $wpdb->get_results($query, OBJECT);
}