I think whatever date helps you stay sober and feels right to you is good.
I agree with TheEnd that it is a gray area.
I recently drank a lot this past summer after 16 years of sobriety. I consider myself someone with both 16 years sobriety and 2 1/2 months sobriety. That works for me. I'm not sure if I would have restarted my count if it had only been one drink.