To include a time and date stamp in the file name when the log file is rotated, in the Log File Name field, add java.text.SimpleDateFormat variables to the file name and surround each variable with percentage (%) characters.
For example, if you enter the following value in the Log File Name field: access_%yyyy%_%MM%_%dd%_%hh%_%mm%.log, the virtual host's HTTP log file will be named: access_yyyy_MM_dd_hh_mm.log. When the server instance rotates the HTTP log file, the rotated file name contains the date stamp. For example, if the server instance rotates the log file on 2 Jan, 2005 at 10:05 AM, the log file that contains the old log messages will be named: access_2005_01_02_10_05.log. If you do not include a time and date stamp, the rotated log files are numbered in order of creation filenamennnnn, where filename is the name configured for the log file. For example, access.log00007
http://e-docs.bea.com/wls/docs90/ConsoleHelp/taskhelp/logging/EnableAndConfigureHTTPLogs.html