Calculate avg difference between dates omitting calculation if certain fields are blank

I am trying to calculate the total average difference between dates, but I want to omit including it in the calculation if certain fields are blank. I feel like I should be able to figure this out, but I have tried everything I can think of. I have attached a very basic sample worksheet due to most of the data being confidential. Basically, I have a "Date Sent for Approval" column and a "Date Closed" column. When I send something for approval I enter the date in the "Date Sent for Approval" column. When it is approved and closed I enter the date in the "Date Closed" column. The problem is that I also have instances where both columns are blank, date closed is blank (because its still pending), or the date sent for approval is blank (because we closed it immediately without approval needed). I don't want any of those previous instances to affect my average calculation. I would like to be able to have on my dashboard a box that says "Days to Close" with the average of the difference between the "Date Closed" and "Date Sent for Approval"

Thank you,
Brooke