I need help with formulas counting dates.

I have open dates and close dates. I need to know the average, minimum and
maximum time a case is open by year. I have figured out the array for the
total period (2002-2006), but can't break it out the same by individual year.

I also am having a problem getting a field to remain blank if no close date
has been entered. Currently it reports 01/00/00 for blanks.

Any/all help appreciated!