Printing currnt time with file

hi,i want to create log file for every hour i use radius. now it islog_file = ${logdir}/radius.logi want like thislog_file = ${logdir}/radius-%Y%m%d:%H.log if i change file name like above one radius create file ' radius-%Y%m%d:%H.log ' i want to create it like 'radius-20120903:08.log' pleae help me in this Regards Qasim