If you don't register your warranty, they will take the date that they "sold" the sets to the local stores. This is from experience and enquiry.
Considering 500D was released in April 2009 in Singapore,
http://www.clubsnap.com/forums/canon/506545-eos-500d-released-sgp.html
and already replaced by 2 models, 550D and 600D, it will be quite obvious even if you register the warranty now which shows Dec 2011.
You will need to prove with a receipt that shows you have bought the 500D recently since most stores have stopped carrying the 500D.
Meaning to say, when they check their system, they can see that this set is more than 2 years old + (the period the store has kept it) - and it is not quite possible any camera store will hold 500D for that long.