How do I find the average approval time?

I have a sheet that our PMs approve invoices and another sheet that copies the rows as soon as they are approved. One column is Date Created (Created Date) and the other is Approved Date (Modified Date). When the row is pulled, it captures the modified date when they approve the row. I need to find the difference between the two rows (Date Created- Approved Date). But I can't create the formula within that row, otherwise it changes the Approved Date column because it is a "Modified Date" column. Then I need to find the average of all the row that constantly get added to this sheet. How would I go about creating the continuous formula without doing it on this sheet? Any suggestions?


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