I know that I have done something similar in the past but I can't recall how, right now . 
I will have to dig through some old applications to see if I can locate the code.

I think it had to do with ResultSets or FormResultSets that match user defined criteria.

But, in the meantime, how about this as a concept?
Set the default sort for the sub form to be by the date, descending order.
Loop through each of the records, looking at the date element.
As long as each record date is newer than 6 months ago, keep the record.
As soon as the first record comes up with date over 6 months old, stop the loop.
Now make the first group of records Visibla and all the others Invisible.

Version 1x has a group of ResultSet commands, including RemoveResultSet.  I don't have access to Sesame or documentation right now.  But you may be able to use that to remove records over 6 months old from the normal result set.  Check the User Manual and Programming Guide.

Version 2x has a stronger group of commands plus XResultSet commands including the powerful XResultSetSearch.

But the solution that I used in the past was using Version 1x, I believe.  Someone else will probably provide a solution for you before I locate mine.  But I will try to find it for you, appreciate your patience.