Sober dates are tricky. I can't tell you how many days I've continued to drink because I didn't like the date for my sober date. Someone's birthday, death, a holiday, all excuses to keep drinking till I got the date "right". I finally settled on 2/13, yep unlucky 13. My therapist told me every day of the year has something funky to it, so I gave up.
To the point, I use my first day sober. I have a sister who uses the day she picked up a white chip, which was several days after her last drink. To each their own.