The format is in 'YYYY-MM-DD' or YYYYMMDD format, depending on whether the function is used in a string or numeric context.
mysql>
mysql> SELECT UTC_DATE(), UTC_DATE() + 0;
+------------+----------------+
| UTC_DATE() | UTC_DATE() + 0 |
+------------+----------------+
| 2007-07-23 | 20070723 |
+------------+----------------+
1 row in set (0.00 sec)
mysql>