How to include only "active" rows in the Oracle Text index.

Is it possible to avoid certain rows to be included in the index? The issue is that the indexed table has from- and to-date field for history reasons. Only rows where sysdate is between from- and to-date or to-date is null should be considered as "active" and hence subject for indexing. Also, when a row gets a to-date less than sysdate, it should be removed from the index.. Not sure how to solve this.... But thanks :-)