I used the first day without drinking as my sober date. Yet I didn't consider myself sober until a couple of months after I worked the AA Twelve Steps with my sponsor, about fourteen months into sobriety. There is no official documentation of such things; it's all very subjective.
It's been my experience that when people do the work, then the "time served" and the milestones become insignificant in comparison to just living one's life. It was important for me to chart the days and months in early recovery as an objective means of measuring my progress, e.g, that I could actually stay sober over time. After, say, one year (your mileage may vary), if I'm still complaining about the things I complained about in my first ninety days, it's time to make a change.