Bamboo makes use of log4j.properties to output logs to <bamboo-install>/logs/catalina.out and <bamboo-home>/logs/atlassian-bamboo.log, making it configurable. However, the option is not available in build logs as the output format is hard-coded:

public static String DATE_FORMAT_STRING = "dd-MMM-yyyy HH:mm:ss";

As a suggestion, make this property configurable.