Here's my little mental gymnastic routine.
I have never lied about my sober time, ever. I drank at 3 months (got blackout drunk, very bad) and changed my sobriety date, was open about it. I then put 6 months together, then had a glass of wine. Somehow it stopped there but it scared the sh!t out of me. Changed my sobriety date, was very open about it. As I come up to what WOULD have been a year had I not drank at 6 months and I worry about non-program, non-family people who know I am in recovery asking how long I have. I worry that in their eyes any work I had done on myself in those original 6 months was totally negated by that glass of wine. My drinking impacted my academic program and I am sometimes freaked out at the idea that someone there will ask me about my time. It has crossed my mind that I should lie about it if that happens.
But because this is considered a very confidential matter at school, no one has even come close to questioning me about anything touching recovery... which makes me think all this worrying I'm doing has a lot more to do with how _I_ feel about that slip than what they _would_ feel about it, if they knew about it.
In talking about this with my sponsor, she has told me that she too has a should-have-been sobriety date that she had to give up due to use of another drug (many decades ago this happened, she has nearly 30 years sober). We plan to have dinner in honor of my should-have-been date, let it go, and refocus on the here and now
But to wind up, I'd rather have someone's opinion of me lowered by having less time than knowing that they thought highly of a liar. I couldn't face myself in the mirror.