I think the level of offense gets magnified in the fishbowl of etiquette discussions. In the real world I rarely see people actually getting upset over this. It's obvious that if someone is checking the time, they're checking it relative to an upcoming obligation; otherwise there's no reason to check the time. If someone does get offended, it's because they consider whatever they have to say more important than wherever the time checker will soon have to go, in which case I have no sympathy for their presumptuousness.Many folks these days seem to be looking for an excuse to be offended. When two of them meet, it can become a competition where each claims they have more of a "right" to be offended. Best to avoid that game. It never ends well.
So the simple courtesy of explaining why you must leave an easily-offended person often helps them to cope with a world where everyone else seems to be against them. Words carry more credibility. Don't assume they'll deduce that your time-checking is benign. They're simply not wired that way
However, in most cases you would really have to make an effort to check the time so conspicuously that it would even register to the other person. A typical watch glance is a discrete, split-second action, whereas most people "rudely" avoid eye contact with their interlocutor for much longer intervals with or without an external pretext. Let's face it, it's rare that we meet people who aren't so preoccupied with their own issues that they can give you their unreserved, undivided attention.
