[rules-users] How to get correct version of rules based on effective date

Hi, We are evaluating Drools to replace a big legacy eligibility system in government sector. One of the critical requirement is to have the system pull correct rule based on a date e.g interest rate was 5% till 12/31/2010 but changed to 6% from 1/1/2011. If we are running rules in 2010 or before, calculation should use 5% else it should use 6%. Rules will run with a user clicking a button and results will be shown to user so probably a StatelessSession will be used. Something like AgendaFilters to filter rules based on a date probably will not be very scalable since over time we'll have lot of different temporal version of rules and all of them will have to be in KnowledgeBase for AgendaFilter to work which will be memory intensive and un-needed.IMHO, we should build KnowledgeBase with correct version of rules in the first place. Any ideas or pointers? Thanks