@ iPix Maxis & Digi staff are supposed to activate the phone for you when you buy it. Now, if you hacktivate it... theoretically you could 'extend' the date...

but Apple thought of that. The sales records are updated online, so if there's ever a dispute about the warranty, they will take the date of sale as the final answer.

tl;dr version : date of purchase. its on the receipt.

I used to work at machines AND Maxis Sunway. so I guess I know what I'm talking about. I can't share all the details here, but lets be clear. Activation is not relevant.
Warranty starts on date of purchase as recorded in partner's POS system & customer's receipt