well secure should be being rotated weekly if not daily. so they should already be in a limited timescale already. On a well managed system, each day /var/log/secure will be compressed at 4am, to secure.1.gz, and a new file created.
as for grepping things regardless, you just need to craft a text string which suits what you want, e.g.
grep 'Sep [4-6]' /var/log/secure
Learn regular expressions, the world is your oyster!