If there's anything I've learned from my 21 years of having a job is that you never tell your boss that you were late. Let them figure that out on their own. I'm not saying you need to lie on your time card/sheet/whatever but you don't need to spend precious time hunting down your boss just to say "Sorry I'm late" which could then lead to a serious talk about how maybe you aren't putting forth the effort into the company anymore and that maybe it's time to consider parting ways. SCREW YOU, TOM!!