man logrotate:
CONFIGURATION FILE
dateext
Archive old versions of log files adding a daily extension like YYYYMMDD instead of simply adding a number. The extension may be configured using the dateformat option.
dateformat format_string
Specify the extension for dateext using the notation similar to strftime(3) function. Only %Y %m %d and %s specifiers are allowed. The default value is -%Y%m%d. Note that also the character separating log name from the extension is part of the dateformat string. The system clock must be set past Sep 9th 2001 for %s to work correctly.