I'm looking to create a calculation in my table that will tell me how many items were sold of a particular sku in the last 30 days. I was thinking of using the count function but I'm not sure how that would work with dates. Any ideas?

There are a couple of ways of doing this. In the first instance I'd look at creating a summary report to generate the information as-needed. You''d create the report by performing a find for the date range, then sorting by SKU then displaying the report layout.

Making this into a constant display is both complex and very processor intensive and it will significantly slow down the database when record counts get large.