Filters for offline sync

I am using Outlook2003 SP2 on a Vista Ultimate system. In order to minimize
the amount of data in my .ost file I applied create a filter on each of my
folders that I take offline. The criteria I used (per our CRM software
provider) was as follows: Received on or after 30 Days Ago. This criteria
has been in place for a few months now and my .ost has continued to grow. The
items that are showing up offline in my folders have received dates (and
modified dates) that seem to be 30 days prior to when I first created the
criteria and applied the filter. So, it seems that the criteria is static and
does not slide forward as the date advances. Is this possible? If so, how can
it be done so it is dynamic? If not, what can be done to correct the problem?