You can use MySQL functions to subtract from either of the date values before comparing. Example as follows.
Subtract 15 days from Expiration Date:
DATE_SUB( `cb_memberexp`, INTERVAL 15 DAY )
Subtract 15 days from Current Date:
DATE_SUB( NOW(), INTERVAL 15 DAY )
Please see the below MySQL documentation for further date function usage.
dev.mysql.com/doc/refman/5.6/en/date-and-time-functions.html
Please note `cb_memberexp` is a CB field created within CB > Field Management so it doesn't exist unless you've created it. It also doesn't have their expiration date unless you setup your CBSubs plans to populate it with expiration date. Alternative is to query _subscriptions table for the expiration date directly.