You'll have to clarify what exactly you're trying to do. The current dashboard has fixed-time displays, so adding a list box won't actually change anything.
If you're trying to change the fixed-time aspects, it's not too hard - they look something like this:
=num(Max({1<PerformanceDate={">=$(=date(today()-6,'YYYY-MM-DD'))"}>}VMCommitted_MB_)/1024,'#,##0')
Simply change the date aspect from today()-6 to e.g. Today()-30, YearStart(Today()), or whatever it is you want displayed.
