The math does add up.
The voice counting misses the 1 the first time, but gets it the second and third.
So he says:
7294531
17294532
17294533
Sayid then estimates the length of the message to about 30 seconds (which pretty much anyone can do).
So that's all you need:
((((17294533*30)/60)/60)/24)/365 = 16.452181316590563165905631659056 = ~16.5 years.
The math is very easy, but doing it in your head can be discussed.
My general theory is either a sentient Island (like pointed to before) or a rip in time (polar bear from the ice age, big animals possibly extinct now), not any elaborate theories yet, but I'll get there