Forgot to mention that you can change the string to any format you want, so for example you can use
if datediff('month',[Parameters].[Date Filter],[Date Truncate])>6 then
elseif datediff('month',[Parameters].[Date Filter],[Date Truncate])<0
then "before slected"
else DATENAME('month',[Date Truncate])+" "+str(year([Date Truncate]))
which looks better than the calc in the attached workbook. Just make sure you sort the Date Calc field by ascending maximum Date Truncate so that the strings are ordered chronologically.
Thanks. I've never thought of using two sheets to do this.