syslog and setting new date format

I am running RHEL7.2. Syslog version 7.4.7. I'd like to change the date format in syslog files from default to:

YY/DOY (Where YY is two-digit year, and DOY is Day Of Year).

I am struggling with the syslog man pages. I know I can create a custom template in syslog.conf, but what format do I use?