You are talking about filtering a list box? If your dimension displayed in the list box is 'Date' and Date field is of date type, I would open the search field by clicking the magnifying class in the caption of the list box and enter
=Date>addmonths(today(),-18)
to filter the last one and a half year of dates.
Or soemthing like
=Date<addmonths(today(),-6) and Date>addmonths(today(),-24)
to filter the date range two years back until 6 months back (1,5 years range).