Currently we can query on relative periods ("This Year", "Last Year" etc.) or absolute date ranges ("between Jan 1st 2012 and Dec 31st 2012").
It's not possible to query on relative dates such as "past 18 months" or "past 30 days". This would be very useful for segmentation.
Please combine your vote to "Add ability to query/report on gifts by last x days/months/years". This is the same thing.