You are saying that Kerry may have been in the IRR and had no duty to appear anywhere unless called up. That seems the most logical to me, since he was so visible in political circles during those years.
So Lt Kerry's active duty was 3 years and 18 days. Lt Bush spent almost as much time on 'active duty' given all his pilot training and subsequent flying hours.
I know this is not a big deal given that we have only 3 weeks to convince as many voters as possible that Kerry is Unfit for Command, but it is one more clue to the character of both men.
Right - however, even while in the IRR he was subject to the UCMJ and Naval procedures - up to and including potential administrative discharge for conduct unbecoming or other behavior detrimental to the naval service.
My opinion stands that it is likely - not necessarily certain - but likely that he was admin sep'd with a general rather than an honorable discharge, due to his post-(did you know he was in)vietnam activities. He could have then petitioned to have this upgraded to Honorable. Given the awards he attained (deserved or not is not the question) this would also likely have occurred. I have seen characterizations based on misconduct that were upgraded after a period of time, so it would not suprise me to see a general upgraded given the documentation the board would have had to review. Not saying I concur just that it happens.