Thanks for the replies, guys. I actually omitted the information that what I really did was initially make a trip to Indonesia (I'm a citizen of Indonesia), transiting via Hong Kong (hence 'Hong Kong trip'). I later on made the trip to Hong Kong (entry on September 20s), so my entry date to HK was much later, so this would not help in proving that I boarded the plane on September 3.
So yes, I do have an entry stamp while entering Jakarta, dated September 4 2013, and I boarded the plane from Toronto on September 3 (to Hong Kong, but they did not stamp my passport during during transit, nor they did any immigration check). Would the (online) ticket, proof of payment, and this entry stamp be enough to prove that I really boarded the plane on September 3 (would they be suspicious that I somehow 'made' the customs stamp a much later entry date, which I of course did not do)?
I wish I had a more definitive proof (such as medical records or something) that I was present in Canada on the third of September, but I don't

I really shouldn't have asked for mobile boarding pass...