I'm not entirely sure. I've tracked my earliest forum post related to ponies (on another forum) back to 06-28-2011, but considering it's only a forum post, it must've been a few days earlier that i became a brony.
I forgot that yesterday was my birthday for joining this forum. This is the only brony forum I've ever taken part in, and I intent to remain loyal here.
Partytime! I think it's a year for me too. The first epsiode I watched was Applebuck Season, while I was buying The Ticket Master on DVD.