For myself personally, that is one reason I do not count days. I have a rough idea of how much clean time I currently have (about 8 months) but I found when I counted I would almost always "fail" right before a "big anniversary", like 30 days, 3 months, etc. I'm still not exactly sure why, I think it was like you said, it was easy to talk myself into saying, "I've been clean for such and such an ammount of time, I'm cured!! I'm sure I could just use a little and it would be OK." I can't tell you the hundereds of times I did that!!
At the same time, I know people with very long term sobriety who can tell you exactly how long they have been sober practaly down to the hour. I guess you just need to find what works for you. Take care.