You have until 1 year after purchase to add AppleCare to the computer. Your best bet is to go to an Apple store or call AppleCare support and give them your serial number and what not and they may be able to lookup the sale date.
ok so i have an aluminum macbook and it is having tons of problems. i want to find out how much time i have left on my applecare warranty. when i go to https://selfsolve.apple.com/GetWarranty.do and enter in all my info it want me to put in my date of purchase... but i don't know when i purchased it... i can't remember. :/