The format is in 'HH:MM:SS' or HHMMSS format, depending on whether the function is used in a string or numeric context.
mysql>
mysql> SELECT UTC_TIME(), UTC_TIME() + 0;
+------------+----------------+
| UTC_TIME() | UTC_TIME() + 0 |
+------------+----------------+
| 02:46:23 | 24623.000000 |
+------------+----------------+
1 row in set (0.00 sec)
mysql>