I think it is a good question. Maybe the way to solve it to create a data entry matrix with a time filter showing only current month and forward. This matrix should not have permission for user to change the filter or expand. Create another matrix – calculation only. It should simply take data from the matrix with entry data using formula. This one will show whole set of data (no filter). So, user will not be able to change past data since he or she cannot see the past data on the entry matrix, but will be able to see whole set of data on calculation matrix.