If I had anything to drink,intentionally then I'd consider myself back at day 1.My period of sobriety would have ended by taking a drink.Of course, I'd still have the period of time previously where I'd been sober and aim to get back again.
For me if I had a drink and didn't go back to day 1 it would be too easy to keep going and do it again and again even though the only person I'd be fooling would be me.
I'd be more concerned about WHY I'd had a drink and what had gone wrong with my recovery.