Excel- Average days between group of dates

I'm trying to use excel to calculate the average frequency of delivery for a set of parts. I have a data set that has two columns- part number and delivery date. I'm trying to figrue out out oftne parts get delivered, on average, in terms of days. I tried using nested ifs like averageif(a2=a2:b9999,datedif(xx)) etc, but to no avail. I'm looking for this:

Input:

Part A    8.1
Part A    8.8
Part A    8.15

Output: Part A Average Delivery - Every 7 Days

etc etc. Any ideas?