For about the first 6 months at my old job, I would arrive at work about 30 minutes early, by virtue of the families daily routine. Since I was there, I'd just walk in and start answering phone calls etc. “My immediate boss pulled me aside fairly early on and said, “You know you don't have to come in early, right? The company doesn't pay for additional voluntary hours.” Obviously covering his ass against me claiming OT or something.
Anyway, one day if finished my work early and decided to take an early 15 minutes. The next morning when I walked in (30 minutes early), that same boss pulled me aside and gave me the, “please explain” for me leaving early and said that if it happened again I would get a warning.
My response, “Oh okay. I'll remember that.”
Every day from then until I left, I would sit in my car with an alarm set and only walk into the building dead on my starting time. Every day I would clock out at, or just a few minutes after, my finish time.