If you bought the oil and filters with a credit card, you can show the date and place the purchase was made and probably even get a reprint of the receipt. I belong to multiple auto rewards programs and can easily look up when, what and where I purchased all my maintenance supplies. All my online purchases from various places also provide purchase histories (Amazon, PureDieselPower, etc). I keep a log book for each of my vehicles with fuel and maintenance entries which should also suffice. I even clip the UPC code from the oil, fuel, and air filters and write a date and mileage on them. If you paid cash for the oil change supplies and don't remember where and when you purchased the items, you might be SOL. All your filters should have date codes stamped on them, but might not help if they were old when purchased. I always buy my auto supplies with the newest date codes.