Average Days Between Date Stamps

I'm trying to complete an excel project for work that keep track of orders from several systems as they come in. Every time I get a new order I put the name of the part ordered in column B4:B5000 and have it set to automatically datestamp that days date in column D4:D5000. The problem I'm running into is that on a separate sheet I want to keep track of the average time between orders while also skipping any blank cells and returning a 0 instead of a div/0 error if the sheet has no orders. For Instance:

Widgets
100
10/15/14

Sprockets
44
10/20/14

Cogs
60
10/25/14

The answer I would be looking for here should be 5 skipping over the blank row. I've scoured the net and tried numerous formula but nothing seems to work.