I am with Rowan, if I chose to have even a sip of wine I would change my sobriety date, how ever if I picked some ones drink by mistake and realized the mistake right away I would not change it. When people in AA speak of their sobriety date it is in regards to thier last drink or first day without a drink.
Slip versus relapse..... just my opinion, but a slip is a mini relapse, anything from a sip to a one night drunk, anything beyond that is a relapse, just my opinion and nothing more.
Remember my opinion and $1.50 may get you or me a cup of coffee.