But if you never register your mac, you don't get warranty right? The warranty starts from the date when you register your mac?
Again, the warranty is tied to the machine, not the person, thus registering will not help extend any existing warranties.
The warranty starts from the day of purchase, not from the day of registering. You do not even need to register a Mac (though I do not know how registering works with
Mac OS X 10.7 Lion and
OS X 10.8 Mountain Lion, but that still does not have to do with the warranty).
If you buy your Mac on 23/12/2011 (dd/mm/yy) and register it on 5/11/2012 (dd/mm/yy), the warranty is still only valid till 22/12/2012 (dd/mm/yy), and the extended warranty (Apple Care Protection Plan), if purchased AND registered within the first year of the Mac being bought, is only valid till 22/12/2014 (dd/mm/yy).
See, as soon as you buy a Mac from Apple, it gets registered in their database as being bought, to automatically start the warranty, as registering a Mac is not necessary (again, I do not know how it is with current OS X versions due to Apple ID excrements). If you buy your Mac from an authorised reseller or other vendor, that reseller or vendor sends its data of Macs being sold (serial numbers and dates) to Apple, thus Apple can put it in their database, again, registering your Mac is not required.
Was that understandable?