I have never seen anything other than the warranty being retroactive to the original purchase date. If you buy from an authorized dealer, its the date of your purchase. If the seller is not an authorized dealer than its the date the seller purchased.
If Apple is unable to determine the date, they will ask you to produce a receipt from an authorized dealer.
The time you have to buy Applecare varies by device.
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Something else is going on here as activation date is not generally correct. Where did you purchase?