That you have to do in the backend.
For example say the Date field in your script is called Calendar,
// Back End Situation
if(Date(Calendar)>='03/10/2011' and Date(Calendar)<='03/17/2011', 'Flag', 'No Flag') as DATE_FLAG
and then reload you will get a flag field in your data model, which you can utilize.
Now, if those selected dates are to be variable, then you will need to define more what exactly is your situation.
Also, I haven't have knowledge if Set Analysis works in Backend.
// In front end i am not ready yet to answer this question.
Because as per my knowledge, set analysis does aggregation function, so how can it set flags for a date range is what I would also be interested in learning.
Hope this helps,