The "blank" warranty card is likely meaningless - when I purchased my Garretts, I registered them online using Garrett's website. When I sold them, I gave the blank warranty cards to the new owners, but like John said, Garrett will know by my electronic registration when the clock started ticking for the warranty (which I disclosed to the buyer). A question might arise if the previous owner never registered the detector - unless the previous owner can provide you with an itemized purchase receipt, Garrett will likely go by the date of manufacture as determined by the serial number. For example, I have an original ProPointer that needed service at some point - I had never registered it, and didn't have a receipt or any recollection of exactly when it was bought, but I knew it was close to or more than 2 years. Garrett had no choice but to go with the manufactured date.
