Where can I find the date/time a void was processed?

If you need to find the dates/times that transactions were voided, you will want to use the Transactions export. 

This export includes a modified_at column which would display the most recent modification that was made to a transaction. Voiding a transaction is considered a modification, so the modified_at column would show you the date that void took place.

This export includes an Invoice_ID column that you can use the identify the invoice the transaction is associated with. You would use the Status column to determine which transactions were voided.