A slip is when you step on a wet spot (alcohol or otherwise) and loose your footing. If you decide to drink, a sip or a gallon; call it what it is, a "decision" to imbibe alcohol.
You don't have to change your sobriety date if you have decided to start rationalizing your behavior to yourself. In fact if just being able to tell others about your "factious" date is important, count each real sober day as two and you will end up being "twice" as sober.
Point is a sobriety date belongs just to you. It is an intensely personal matter and you should be able to look yourself in the eye and know the truth.
SIMPLE AS THAT!
Jon