My boss wants me at work by 8:00 a.m. Outside of rush hour, my drive to work takes about 15 minutes. During rush hour the commute time can vary greatly. (Public transporation isn't a reasonable option, as I'd have to change buses a couple times, so that would take hours.) I allow at least 30 minutes to drive to work and am usually at my desk by 7:55. Occasionally (about once per month) traffic is exceptionally bad and it is 8:05 or 8:10 before I'm at my desk - it has never been later than that. My boss - who gets in at 7:00 a.m., always makes some sort of comment when I'm here later than 8:00. It has not (yet) been threatening, but he seems to be making a point of letting me know that he noticed I'm late. My response has always been along the lines of "Traffic was especially bad today. Sorry I was late."
Extenuating (and somewhat aggravating) factors: With an 8:00 a.m. start time I'm expected to stay until 5:00 p.m. I'm always at work until at least then, and often later. When I'm late I make sure I stay in the office at least that much longer. Since my boss gets to work at 7:00 a.m., he leaves at 4:00 p.m., and he never stays after that time (although on a couple occasions he has come back into the office to help fix problems), so he doesn't see how late I'm at work. Also, I often work Saturdays to catch up on things that got pushed aside during the week; I've never seen him at work on a Saturday.
It seems to me my boss' expectations that I never be late are unreasonable. Thus my question is How can I diplomatically tell him that I'm doing the best I can and that he should overlook these occasional, uncontrollable times when I'm a few minutes late?
Edit to add some information (some already noted in comments):
I have never been told that arriving after 8:00 is a real problem, my boss just makes a point of saying something when it happens, which gets on my nerves. Also, I didn't know my supervisor starts at 7:00 until after I started the job.
Part of my job is to help people in other departments when they are having trouble. It depends on the department, but most of these people are supposed to work 8:00 to 5:00, although some start as early as 6:30. If I'm not around, they can call my boss.
My boss has a reputation of being an unpleasant person (in fact, other than my issue here, I generally get along with him better than most people in the organization), so people almost always call me and not him if they need help.
We don't have any sort of guarantee of immediate response. In fact there have been several times when I have been at someone else's desk providing assistance when another call comes in. Some of these people have told me they just waited for me to respond, instead of calling my boss.
While I've been lucky enough to avoid being egregiously late in the few months I've been on the job, traffic in this area is so bad that I don't think there is any time I could leave home and guarantee I'd be at work by 8:00.
Due to home responsibilities, starting my commute significantly earlier than I do isn't possible.