Honestly, it isn't a big deal. My home group refers to 30, 60, 90 days and then six months, and a year. The only reason anyone MIGHT question anything (and it shouldn't be in front of the group) is that some people would go up to get a chip when their anniversary isn't until the following week. In those instances we usually remind people that we don't celebrate (even 20 years) until the date comes. (Again, though, this should be a private discussion, and NOT a "scold").
Another perennial dilemma some people have is whether they count from the day of their last drink or their first day without a drink. I know people who count both ways. That doesn't matter, either. You can celebrate either day.
Getting bogged down in that sort of thing sort of defeats the whole purpose.