Average by Month of Averages

I took over an AR position from someone who did not keep track of what invoices were outstanding, what was paid, etc. It was a mess. I created a spreadsheet to track every invoice and cc purchase from the time I received it until the time I gave it to the controller to pay. It is a MILLION times better than it was, but bc of the hurricane and mail sorting issues, our mail is delayed by weeks sometimes getting from our mailbox to the vendors. I NEED to show my boss that the problem is not me.

I took my spreadsheet and added columns at the end to measure the average time it takes in each step of the process. Those are the formulas labelled F and G on the attachment. I do not have all the info I need for the earlier invoices bc of my predecessor. So there are lots of rows where the formula result is unusable.

I figured out how to get the averages of each step of the process using only the actual numbers and omitting the blanks and weird results I get from the missing info. Those formulas are A - E are at the bottom of all the data (only a portion is attached).

I CANNOT figure out how to take the overall average and break it down by months. I want to show that the age of the invoices is less than 5 days in December instead of 100 it was when I took over the position, for example.

Any help would be appreciated. Thanks!

On the attachment I had wrote the formulas that are in the cells that gave the shown results.