And this indeed could pose a problem. However, is it true that USCIS will check through the system to verify exactly the dates you "actually" left and returned as opposed to what you tell them?
For eg, if they rely on passport stamps (which sometimes don't get stamped) then we have unreliable information, and sometimes passports expire and are lost/stolen etc. Now when the GC is swiped, obviously it creates an arrival record, but no departure record at least on the GC, so of course there is some grey area.
If one were to fudge the departure date by one day, so making the whole trip six months minus one, what kinds of cross-checking, verification etc. should I expect USCIS to run at the time of N400 application?
Personally, I think exact six months should not be a problem, but this is USCIS we are talking about. Is it worth checking with an attorney if my clock is already reset or if I can continue proceeding with the premise that I will be N400 eligible when the time comes?
To some other posters, I am not concerned at all about re-admission into the US at the end of six months, since I am already admitted, this question is strictly from the perspective of N400 eligibility.
Exactly six months is enough to put you in trouble. The Guide to Naturalization repeatedly refers to "6 months or longer", and the N-400 has a column that asks "Did trip last six months or more?".