Which export can I use to find past and future renewal dates?

If you are looking for future renewal dates, your best bet is to download the Subscriptions export (Status: Renewing, Time Range: All Time) and then once downloaded, use the "current_period_ends_at" column to determine when the next renewal date will be. 

To find past renewal dates, what you can do is download the Invoices - Summary export (Status: Closed, Time Range: All Time) and then use the "invoice_type" column to filter out all types except for renewals. You'll be left with a list of invoices for renewals only.