I use the last day I drank. For a while I felt sort of guilty about that but it makes sense to me.
I went to an AA meeting the night before. I was drunk when I called AA and when I went to the meeting.
I got up early the next day and felt like hell. I drank. But then the clarity I had from the night before hit me. Here I was doing the same thing. I was going back, again. I was going to throw away the gift I got. The clarity was a gift and I was going to drink it away.
I quit drinking. I ate something and went to bed early at four in the afternoon. I have not had a drink since and I have had no desire to drink.
That was the day I quit and that is the day I use as my sobriety date.